1PERLGUTS(1) Perl Programmers Reference Guide PERLGUTS(1)
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6 perlguts - Introduction to the Perl API
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9 This document attempts to describe how to use the Perl API, as well as
10 to provide some info on the basic workings of the Perl core. It is far
11 from complete and probably contains many errors. Please refer any
12 questions or comments to the author below.
13
15 Datatypes
16 Perl has three typedefs that handle Perl's three main data types:
17 SV Scalar Value
19 AV Array Value
20 HV Hash Value
21 Each typedef has specific routines that manipulate the various data
23 types.
24 What is an "IV"?
26 Perl uses a special typedef IV which is a simple signed integer type
27 that is guaranteed to be large enough to hold a pointer (as well as an
28 integer). Additionally, there is the UV, which is simply an unsigned
29 IV.
30 Perl also uses two special typedefs, I32 and I16, which will always be
32 at least 32-bits and 16-bits long, respectively. (Again, there are U32
33 and U16, as well.) They will usually be exactly 32 and 16 bits long,
34 but on Crays they will both be 64 bits.
35 Working with SVs
37 An SV can be created and loaded with one command. There are five types
38 of values that can be loaded: an integer value (IV), an unsigned
39 integer value (UV), a double (NV), a string (PV), and another scalar
40 (SV). ("PV" stands for "Pointer Value". You might think that it is
41 misnamed because it is described as pointing only to strings. However,
42 it is possible to have it point to other things. For example, it could
43 point to an array of UVs. But, using it for non-strings requires care,
44 as the underlying assumption of much of the internals is that PVs are
45 just for strings. Often, for example, a trailing "NUL" is tacked on
46 automatically. The non-string use is documented only in this
47 paragraph.)
48 The seven routines are:
50 SV* newSViv(IV);
52 SV* newSVuv(UV);
53 SV* newSVnv(double);
54 SV* newSVpv(const char*, STRLEN);
55 SV* newSVpvn(const char*, STRLEN);
56 SV* newSVpvf(const char*, ...);
57 SV* newSVsv(SV*);
58 "STRLEN" is an integer type ("Size_t", usually defined as "size_t" in
60 config.h) guaranteed to be large enough to represent the size of any
61 string that perl can handle.
62 In the unlikely case of a SV requiring more complex initialization, you
64 can create an empty SV with newSV(len). If "len" is 0 an empty SV of
65 type NULL is returned, else an SV of type PV is returned with len + 1
66 (for the "NUL") bytes of storage allocated, accessible via SvPVX. In
67 both cases the SV has the undef value.
68 SV *sv = newSV(0); /* no storage allocated */
70 SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage
71 * allocated */
72 To change the value of an already-existing SV, there are eight
74 routines:
75 void sv_setiv(SV*, IV);
77 void sv_setuv(SV*, UV);
78 void sv_setnv(SV*, double);
79 void sv_setpv(SV*, const char*);
80 void sv_setpvn(SV*, const char*, STRLEN)
81 void sv_setpvf(SV*, const char*, ...);
82 void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *,
83 SV **, Size_t, bool *);
84 void sv_setsv(SV*, SV*);
85 Notice that you can choose to specify the length of the string to be
87 assigned by using "sv_setpvn", "newSVpvn", or "newSVpv", or you may
88 allow Perl to calculate the length by using "sv_setpv" or by specifying
89 0 as the second argument to "newSVpv". Be warned, though, that Perl
90 will determine the string's length by using "strlen", which depends on
91 the string terminating with a "NUL" character, and not otherwise
92 containing NULs.
93 The arguments of "sv_setpvf" are processed like "sprintf", and the
95 formatted output becomes the value.
96 "sv_vsetpvfn" is an analogue of "vsprintf", but it allows you to
98 specify either a pointer to a variable argument list or the address and
99 length of an array of SVs. The last argument points to a boolean; on
100 return, if that boolean is true, then locale-specific information has
101 been used to format the string, and the string's contents are therefore
102 untrustworthy (see perlsec). This pointer may be NULL if that
103 information is not important. Note that this function requires you to
104 specify the length of the format.
105 The "sv_set*()" functions are not generic enough to operate on values
107 that have "magic". See "Magic Virtual Tables" later in this document.
108 All SVs that contain strings should be terminated with a "NUL"
110 character. If it is not "NUL"-terminated there is a risk of core dumps
111 and corruptions from code which passes the string to C functions or
112 system calls which expect a "NUL"-terminated string. Perl's own
113 functions typically add a trailing "NUL" for this reason.
114 Nevertheless, you should be very careful when you pass a string stored
115 in an SV to a C function or system call.
116 To access the actual value that an SV points to, you can use the
118 macros:
119 SvIV(SV*)
121 SvUV(SV*)
122 SvNV(SV*)
123 SvPV(SV*, STRLEN len)
124 SvPV_nolen(SV*)
125 which will automatically coerce the actual scalar type into an IV, UV,
127 double, or string.
128 In the "SvPV" macro, the length of the string returned is placed into
130 the variable "len" (this is a macro, so you do not use &len). If you
131 do not care what the length of the data is, use the "SvPV_nolen" macro.
132 Historically the "SvPV" macro with the global variable "PL_na" has been
133 used in this case. But that can be quite inefficient because "PL_na"
134 must be accessed in thread-local storage in threaded Perl. In any
135 case, remember that Perl allows arbitrary strings of data that may both
136 contain NULs and might not be terminated by a "NUL".
137 Also remember that C doesn't allow you to safely say "foo(SvPV(s, len),
139 len);". It might work with your compiler, but it won't work for
140 everyone. Break this sort of statement up into separate assignments:
141 SV *s;
143 STRLEN len;
144 char *ptr;
145 ptr = SvPV(s, len);
146 foo(ptr, len);
147 If you want to know if the scalar value is TRUE, you can use:
149 SvTRUE(SV*)
151 Although Perl will automatically grow strings for you, if you need to
153 force Perl to allocate more memory for your SV, you can use the macro
154 SvGROW(SV*, STRLEN newlen)
156 which will determine if more memory needs to be allocated. If so, it
158 will call the function "sv_grow". Note that "SvGROW" can only
159 increase, not decrease, the allocated memory of an SV and that it does
160 not automatically add space for the trailing "NUL" byte (perl's own
161 string functions typically do "SvGROW(sv, len + 1)").
162 If you want to write to an existing SV's buffer and set its value to a
164 string, use SvPV_force() or one of its variants to force the SV to be a
165 PV. This will remove any of various types of non-stringness from the
166 SV while preserving the content of the SV in the PV. This can be used,
167 for example, to append data from an API function to a buffer without
168 extra copying:
169 (void)SvPVbyte_force(sv, len);
171 s = SvGROW(sv, len + needlen + 1);
172 /* something that modifies up to needlen bytes at s+len, but
173 modifies newlen bytes
174 eg. newlen = read(fd, s + len, needlen);
175 ignoring errors for these examples
176 */
177 s[len + newlen] = '\0';
178 SvCUR_set(sv, len + newlen);
179 SvUTF8_off(sv);
180 SvSETMAGIC(sv);
181 If you already have the data in memory or if you want to keep your code
183 simple, you can use one of the sv_cat*() variants, such as sv_catpvn().
184 If you want to insert anywhere in the string you can use sv_insert() or
185 sv_insert_flags().
186 If you don't need the existing content of the SV, you can avoid some
188 copying with:
189 SvPVCLEAR(sv);
191 s = SvGROW(sv, needlen + 1);
192 /* something that modifies up to needlen bytes at s, but modifies
193 newlen bytes
194 eg. newlen = read(fd, s. needlen);
195 */
196 s[newlen] = '\0';
197 SvCUR_set(sv, newlen);
198 SvPOK_only(sv); /* also clears SVf_UTF8 */
199 SvSETMAGIC(sv);
200 Again, if you already have the data in memory or want to avoid the
202 complexity of the above, you can use sv_setpvn().
203 If you have a buffer allocated with Newx() and want to set that as the
205 SV's value, you can use sv_usepvn_flags(). That has some requirements
206 if you want to avoid perl re-allocating the buffer to fit the trailing
207 NUL:
208 Newx(buf, somesize+1, char);
210 /* ... fill in buf ... */
211 buf[somesize] = '\0';
212 sv_usepvn_flags(sv, buf, somesize, SV_SMAGIC | SV_HAS_TRAILING_NUL);
213 /* buf now belongs to perl, don't release it */
214 If you have an SV and want to know what kind of data Perl thinks is
216 stored in it, you can use the following macros to check the type of SV
217 you have.
218 SvIOK(SV*)
220 SvNOK(SV*)
221 SvPOK(SV*)
222 You can get and set the current length of the string stored in an SV
224 with the following macros:
225 SvCUR(SV*)
227 SvCUR_set(SV*, I32 val)
228 You can also get a pointer to the end of the string stored in the SV
230 with the macro:
231 SvEND(SV*)
233 But note that these last three macros are valid only if "SvPOK()" is
235 true.
236 If you want to append something to the end of string stored in an
238 "SV*", you can use the following functions:
239 void sv_catpv(SV*, const char*);
241 void sv_catpvn(SV*, const char*, STRLEN);
242 void sv_catpvf(SV*, const char*, ...);
243 void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **,
244 I32, bool);
245 void sv_catsv(SV*, SV*);
246 The first function calculates the length of the string to be appended
248 by using "strlen". In the second, you specify the length of the string
249 yourself. The third function processes its arguments like "sprintf"
250 and appends the formatted output. The fourth function works like
251 "vsprintf". You can specify the address and length of an array of SVs
252 instead of the va_list argument. The fifth function extends the string
253 stored in the first SV with the string stored in the second SV. It
254 also forces the second SV to be interpreted as a string.
255 The "sv_cat*()" functions are not generic enough to operate on values
257 that have "magic". See "Magic Virtual Tables" later in this document.
258 If you know the name of a scalar variable, you can get a pointer to its
260 SV by using the following:
261 SV* get_sv("package::varname", 0);
263 This returns NULL if the variable does not exist.
265 If you want to know if this variable (or any other SV) is actually
267 "defined", you can call:
268 SvOK(SV*)
270 The scalar "undef" value is stored in an SV instance called
272 "PL_sv_undef".
273 Its address can be used whenever an "SV*" is needed. Make sure that
275 you don't try to compare a random sv with &PL_sv_undef. For example
276 when interfacing Perl code, it'll work correctly for:
277 foo(undef);
279 But won't work when called as:
281 $x = undef;
283 foo($x);
284 So to repeat always use SvOK() to check whether an sv is defined.
286 Also you have to be careful when using &PL_sv_undef as a value in AVs
288 or HVs (see "AVs, HVs and undefined values").
289 There are also the two values "PL_sv_yes" and "PL_sv_no", which contain
291 boolean TRUE and FALSE values, respectively. Like "PL_sv_undef", their
292 addresses can be used whenever an "SV*" is needed.
293 Do not be fooled into thinking that "(SV *) 0" is the same as
295 &PL_sv_undef. Take this code:
296 SV* sv = (SV*) 0;
298 if (I-am-to-return-a-real-value) {
299 sv = sv_2mortal(newSViv(42));
300 }
301 sv_setsv(ST(0), sv);
302 This code tries to return a new SV (which contains the value 42) if it
304 should return a real value, or undef otherwise. Instead it has
305 returned a NULL pointer which, somewhere down the line, will cause a
306 segmentation violation, bus error, or just weird results. Change the
307 zero to &PL_sv_undef in the first line and all will be well.
308 To free an SV that you've created, call "SvREFCNT_dec(SV*)". Normally
310 this call is not necessary (see "Reference Counts and Mortality").
311 Offsets
313 Perl provides the function "sv_chop" to efficiently remove characters
314 from the beginning of a string; you give it an SV and a pointer to
315 somewhere inside the PV, and it discards everything before the pointer.
316 The efficiency comes by means of a little hack: instead of actually
317 removing the characters, "sv_chop" sets the flag "OOK" (offset OK) to
318 signal to other functions that the offset hack is in effect, and it
319 moves the PV pointer (called "SvPVX") forward by the number of bytes
320 chopped off, and adjusts "SvCUR" and "SvLEN" accordingly. (A portion
321 of the space between the old and new PV pointers is used to store the
322 count of chopped bytes.)
323 Hence, at this point, the start of the buffer that we allocated lives
325 at "SvPVX(sv) - SvIV(sv)" in memory and the PV pointer is pointing into
326 the middle of this allocated storage.
327 This is best demonstrated by example. Normally copy-on-write will
329 prevent the substitution from operator from using this hack, but if you
330 can craft a string for which copy-on-write is not possible, you can see
331 it in play. In the current implementation, the final byte of a string
332 buffer is used as a copy-on-write reference count. If the buffer is
333 not big enough, then copy-on-write is skipped. First have a look at an
334 empty string:
335 % ./perl -Ilib -MDevel::Peek -le '$a=""; $a .= ""; Dump $a'
337 SV = PV(0x7ffb7c008a70) at 0x7ffb7c030390
338 REFCNT = 1
339 FLAGS = (POK,pPOK)
340 PV = 0x7ffb7bc05b50 ""\0
341 CUR = 0
342 LEN = 10
343 Notice here the LEN is 10. (It may differ on your platform.) Extend
345 the length of the string to one less than 10, and do a substitution:
346 % ./perl -Ilib -MDevel::Peek -le '$a=""; $a.="123456789"; $a=~s/.//; \
348 Dump($a)'
349 SV = PV(0x7ffa04008a70) at 0x7ffa04030390
350 REFCNT = 1
351 FLAGS = (POK,OOK,pPOK)
352 OFFSET = 1
353 PV = 0x7ffa03c05b61 ( "\1" . ) "23456789"\0
354 CUR = 8
355 LEN = 9
356 Here the number of bytes chopped off (1) is shown next as the OFFSET.
358 The portion of the string between the "real" and the "fake" beginnings
359 is shown in parentheses, and the values of "SvCUR" and "SvLEN" reflect
360 the fake beginning, not the real one. (The first character of the
361 string buffer happens to have changed to "\1" here, not "1", because
362 the current implementation stores the offset count in the string
363 buffer. This is subject to change.)
364 Something similar to the offset hack is performed on AVs to enable
366 efficient shifting and splicing off the beginning of the array; while
367 "AvARRAY" points to the first element in the array that is visible from
368 Perl, "AvALLOC" points to the real start of the C array. These are
369 usually the same, but a "shift" operation can be carried out by
370 increasing "AvARRAY" by one and decreasing "AvFILL" and "AvMAX".
371 Again, the location of the real start of the C array only comes into
372 play when freeing the array. See "av_shift" in av.c.
373 What's Really Stored in an SV?
375 Recall that the usual method of determining the type of scalar you have
376 is to use "Sv*OK" macros. Because a scalar can be both a number and a
377 string, usually these macros will always return TRUE and calling the
378 "Sv*V" macros will do the appropriate conversion of string to
379 integer/double or integer/double to string.
380 If you really need to know if you have an integer, double, or string
382 pointer in an SV, you can use the following three macros instead:
383 SvIOKp(SV*)
385 SvNOKp(SV*)
386 SvPOKp(SV*)
387 These will tell you if you truly have an integer, double, or string
389 pointer stored in your SV. The "p" stands for private.
390 There are various ways in which the private and public flags may
392 differ. For example, in perl 5.16 and earlier a tied SV may have a
393 valid underlying value in the IV slot (so SvIOKp is true), but the data
394 should be accessed via the FETCH routine rather than directly, so SvIOK
395 is false. (In perl 5.18 onwards, tied scalars use the flags the same
396 way as untied scalars.) Another is when numeric conversion has
397 occurred and precision has been lost: only the private flag is set on
398 'lossy' values. So when an NV is converted to an IV with loss, SvIOKp,
399 SvNOKp and SvNOK will be set, while SvIOK wont be.
400 In general, though, it's best to use the "Sv*V" macros.
402 Working with AVs
404 There are two ways to create and load an AV. The first method creates
405 an empty AV:
406 AV* newAV();
408 The second method both creates the AV and initially populates it with
410 SVs:
411 AV* av_make(SSize_t num, SV **ptr);
413 The second argument points to an array containing "num" "SV*"'s. Once
415 the AV has been created, the SVs can be destroyed, if so desired.
416 Once the AV has been created, the following operations are possible on
418 it:
419 void av_push(AV*, SV*);
421 SV* av_pop(AV*);
422 SV* av_shift(AV*);
423 void av_unshift(AV*, SSize_t num);
424 These should be familiar operations, with the exception of
426 "av_unshift". This routine adds "num" elements at the front of the
427 array with the "undef" value. You must then use "av_store" (described
428 below) to assign values to these new elements.
429 Here are some other functions:
431 SSize_t av_top_index(AV*);
433 SV** av_fetch(AV*, SSize_t key, I32 lval);
434 SV** av_store(AV*, SSize_t key, SV* val);
435 The "av_top_index" function returns the highest index value in an array
437 (just like $#array in Perl). If the array is empty, -1 is returned.
438 The "av_fetch" function returns the value at index "key", but if "lval"
439 is non-zero, then "av_fetch" will store an undef value at that index.
440 The "av_store" function stores the value "val" at index "key", and does
441 not increment the reference count of "val". Thus the caller is
442 responsible for taking care of that, and if "av_store" returns NULL,
443 the caller will have to decrement the reference count to avoid a memory
444 leak. Note that "av_fetch" and "av_store" both return "SV**"'s, not
445 "SV*"'s as their return value.
446 A few more:
448 void av_clear(AV*);
450 void av_undef(AV*);
451 void av_extend(AV*, SSize_t key);
452 The "av_clear" function deletes all the elements in the AV* array, but
454 does not actually delete the array itself. The "av_undef" function
455 will delete all the elements in the array plus the array itself. The
456 "av_extend" function extends the array so that it contains at least
457 "key+1" elements. If "key+1" is less than the currently allocated
458 length of the array, then nothing is done.
459 If you know the name of an array variable, you can get a pointer to its
461 AV by using the following:
462 AV* get_av("package::varname", 0);
464 This returns NULL if the variable does not exist.
466 See "Understanding the Magic of Tied Hashes and Arrays" for more
468 information on how to use the array access functions on tied arrays.
469 Working with HVs
471 To create an HV, you use the following routine:
472 HV* newHV();
474 Once the HV has been created, the following operations are possible on
476 it:
477 SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
479 SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
480 The "klen" parameter is the length of the key being passed in (Note
482 that you cannot pass 0 in as a value of "klen" to tell Perl to measure
483 the length of the key). The "val" argument contains the SV pointer to
484 the scalar being stored, and "hash" is the precomputed hash value (zero
485 if you want "hv_store" to calculate it for you). The "lval" parameter
486 indicates whether this fetch is actually a part of a store operation,
487 in which case a new undefined value will be added to the HV with the
488 supplied key and "hv_fetch" will return as if the value had already
489 existed.
490 Remember that "hv_store" and "hv_fetch" return "SV**"'s and not just
492 "SV*". To access the scalar value, you must first dereference the
493 return value. However, you should check to make sure that the return
494 value is not NULL before dereferencing it.
495 The first of these two functions checks if a hash table entry exists,
497 and the second deletes it.
498 bool hv_exists(HV*, const char* key, U32 klen);
500 SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
501 If "flags" does not include the "G_DISCARD" flag then "hv_delete" will
503 create and return a mortal copy of the deleted value.
504 And more miscellaneous functions:
506 void hv_clear(HV*);
508 void hv_undef(HV*);
509 Like their AV counterparts, "hv_clear" deletes all the entries in the
511 hash table but does not actually delete the hash table. The "hv_undef"
512 deletes both the entries and the hash table itself.
513 Perl keeps the actual data in a linked list of structures with a
515 typedef of HE. These contain the actual key and value pointers (plus
516 extra administrative overhead). The key is a string pointer; the value
517 is an "SV*". However, once you have an "HE*", to get the actual key
518 and value, use the routines specified below.
519 I32 hv_iterinit(HV*);
521 /* Prepares starting point to traverse hash table */
522 HE* hv_iternext(HV*);
523 /* Get the next entry, and return a pointer to a
524 structure that has both the key and value */
525 char* hv_iterkey(HE* entry, I32* retlen);
526 /* Get the key from an HE structure and also return
527 the length of the key string */
528 SV* hv_iterval(HV*, HE* entry);
529 /* Return an SV pointer to the value of the HE
530 structure */
531 SV* hv_iternextsv(HV*, char** key, I32* retlen);
532 /* This convenience routine combines hv_iternext,
533 hv_iterkey, and hv_iterval. The key and retlen
534 arguments are return values for the key and its
535 length. The value is returned in the SV* argument */
536 If you know the name of a hash variable, you can get a pointer to its
538 HV by using the following:
539 HV* get_hv("package::varname", 0);
541 This returns NULL if the variable does not exist.
543 The hash algorithm is defined in the "PERL_HASH" macro:
545 PERL_HASH(hash, key, klen)
547 The exact implementation of this macro varies by architecture and
549 version of perl, and the return value may change per invocation, so the
550 value is only valid for the duration of a single perl process.
551 See "Understanding the Magic of Tied Hashes and Arrays" for more
553 information on how to use the hash access functions on tied hashes.
554 Hash API Extensions
556 Beginning with version 5.004, the following functions are also
557 supported:
558 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
560 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
561 bool hv_exists_ent (HV* tb, SV* key, U32 hash);
563 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
564 SV* hv_iterkeysv (HE* entry);
566 Note that these functions take "SV*" keys, which simplifies writing of
568 extension code that deals with hash structures. These functions also
569 allow passing of "SV*" keys to "tie" functions without forcing you to
570 stringify the keys (unlike the previous set of functions).
571 They also return and accept whole hash entries ("HE*"), making their
573 use more efficient (since the hash number for a particular string
574 doesn't have to be recomputed every time). See perlapi for detailed
575 descriptions.
576 The following macros must always be used to access the contents of hash
578 entries. Note that the arguments to these macros must be simple
579 variables, since they may get evaluated more than once. See perlapi
580 for detailed descriptions of these macros.
581 HePV(HE* he, STRLEN len)
583 HeVAL(HE* he)
584 HeHASH(HE* he)
585 HeSVKEY(HE* he)
586 HeSVKEY_force(HE* he)
587 HeSVKEY_set(HE* he, SV* sv)
588 These two lower level macros are defined, but must only be used when
590 dealing with keys that are not "SV*"s:
591 HeKEY(HE* he)
593 HeKLEN(HE* he)
594 Note that both "hv_store" and "hv_store_ent" do not increment the
596 reference count of the stored "val", which is the caller's
597 responsibility. If these functions return a NULL value, the caller
598 will usually have to decrement the reference count of "val" to avoid a
599 memory leak.
600 AVs, HVs and undefined values
602 Sometimes you have to store undefined values in AVs or HVs. Although
603 this may be a rare case, it can be tricky. That's because you're used
604 to using &PL_sv_undef if you need an undefined SV.
605 For example, intuition tells you that this XS code:
607 AV *av = newAV();
609 av_store( av, 0, &PL_sv_undef );
610 is equivalent to this Perl code:
612 my @av;
614 $av[0] = undef;
615 Unfortunately, this isn't true. In perl 5.18 and earlier, AVs use
617 &PL_sv_undef as a marker for indicating that an array element has not
618 yet been initialized. Thus, "exists $av[0]" would be true for the
619 above Perl code, but false for the array generated by the XS code. In
620 perl 5.20, storing &PL_sv_undef will create a read-only element,
621 because the scalar &PL_sv_undef itself is stored, not a copy.
622 Similar problems can occur when storing &PL_sv_undef in HVs:
624 hv_store( hv, "key", 3, &PL_sv_undef, 0 );
626 This will indeed make the value "undef", but if you try to modify the
628 value of "key", you'll get the following error:
629 Modification of non-creatable hash value attempted
631 In perl 5.8.0, &PL_sv_undef was also used to mark placeholders in
633 restricted hashes. This caused such hash entries not to appear when
634 iterating over the hash or when checking for the keys with the
635 "hv_exists" function.
636 You can run into similar problems when you store &PL_sv_yes or
638 &PL_sv_no into AVs or HVs. Trying to modify such elements will give
639 you the following error:
640 Modification of a read-only value attempted
642 To make a long story short, you can use the special variables
644 &PL_sv_undef, &PL_sv_yes and &PL_sv_no with AVs and HVs, but you have
645 to make sure you know what you're doing.
646 Generally, if you want to store an undefined value in an AV or HV, you
648 should not use &PL_sv_undef, but rather create a new undefined value
649 using the "newSV" function, for example:
650 av_store( av, 42, newSV(0) );
652 hv_store( hv, "foo", 3, newSV(0), 0 );
653 References
655 References are a special type of scalar that point to other data types
656 (including other references).
657 To create a reference, use either of the following functions:
659 SV* newRV_inc((SV*) thing);
661 SV* newRV_noinc((SV*) thing);
662 The "thing" argument can be any of an "SV*", "AV*", or "HV*". The
664 functions are identical except that "newRV_inc" increments the
665 reference count of the "thing", while "newRV_noinc" does not. For
666 historical reasons, "newRV" is a synonym for "newRV_inc".
667 Once you have a reference, you can use the following macro to
669 dereference the reference:
670 SvRV(SV*)
672 then call the appropriate routines, casting the returned "SV*" to
674 either an "AV*" or "HV*", if required.
675 To determine if an SV is a reference, you can use the following macro:
677 SvROK(SV*)
679 To discover what type of value the reference refers to, use the
681 following macro and then check the return value.
682 SvTYPE(SvRV(SV*))
684 The most useful types that will be returned are:
686 < SVt_PVAV Scalar
688 SVt_PVAV Array
689 SVt_PVHV Hash
690 SVt_PVCV Code
691 SVt_PVGV Glob (possibly a file handle)
692 See "svtype" in perlapi for more details.
694 Blessed References and Class Objects
696 References are also used to support object-oriented programming. In
697 perl's OO lexicon, an object is simply a reference that has been
698 blessed into a package (or class). Once blessed, the programmer may
699 now use the reference to access the various methods in the class.
700 A reference can be blessed into a package with the following function:
702 SV* sv_bless(SV* sv, HV* stash);
704 The "sv" argument must be a reference value. The "stash" argument
706 specifies which class the reference will belong to. See "Stashes and
707 Globs" for information on converting class names into stashes.
708 /* Still under construction */
710 The following function upgrades rv to reference if not already one.
712 Creates a new SV for rv to point to. If "classname" is non-null, the
713 SV is blessed into the specified class. SV is returned.
714 SV* newSVrv(SV* rv, const char* classname);
716 The following three functions copy integer, unsigned integer or double
718 into an SV whose reference is "rv". SV is blessed if "classname" is
719 non-null.
720 SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
722 SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
723 SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
724 The following function copies the pointer value (the address, not the
726 string!) into an SV whose reference is rv. SV is blessed if
727 "classname" is non-null.
728 SV* sv_setref_pv(SV* rv, const char* classname, void* pv);
730 The following function copies a string into an SV whose reference is
732 "rv". Set length to 0 to let Perl calculate the string length. SV is
733 blessed if "classname" is non-null.
734 SV* sv_setref_pvn(SV* rv, const char* classname, char* pv,
736 STRLEN length);
737 The following function tests whether the SV is blessed into the
739 specified class. It does not check inheritance relationships.
740 int sv_isa(SV* sv, const char* name);
742 The following function tests whether the SV is a reference to a blessed
744 object.
745 int sv_isobject(SV* sv);
747 The following function tests whether the SV is derived from the
749 specified class. SV can be either a reference to a blessed object or a
750 string containing a class name. This is the function implementing the
751 "UNIVERSAL::isa" functionality.
752 bool sv_derived_from(SV* sv, const char* name);
754 To check if you've got an object derived from a specific class you have
756 to write:
757 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
759 Creating New Variables
761 To create a new Perl variable with an undef value which can be accessed
762 from your Perl script, use the following routines, depending on the
763 variable type.
764 SV* get_sv("package::varname", GV_ADD);
766 AV* get_av("package::varname", GV_ADD);
767 HV* get_hv("package::varname", GV_ADD);
768 Notice the use of GV_ADD as the second parameter. The new variable can
770 now be set, using the routines appropriate to the data type.
771 There are additional macros whose values may be bitwise OR'ed with the
773 "GV_ADD" argument to enable certain extra features. Those bits are:
774 GV_ADDMULTI
776 Marks the variable as multiply defined, thus preventing the:
777 Name <varname> used only once: possible typo
779 warning.
781 GV_ADDWARN
783 Issues the warning:
784 Had to create <varname> unexpectedly
786 if the variable did not exist before the function was called.
788 If you do not specify a package name, the variable is created in the
790 current package.
791 Reference Counts and Mortality
793 Perl uses a reference count-driven garbage collection mechanism. SVs,
794 AVs, or HVs (xV for short in the following) start their life with a
795 reference count of 1. If the reference count of an xV ever drops to 0,
796 then it will be destroyed and its memory made available for reuse. At
797 the most basic internal level, reference counts can be manipulated with
798 the following macros:
799 int SvREFCNT(SV* sv);
801 SV* SvREFCNT_inc(SV* sv);
802 void SvREFCNT_dec(SV* sv);
803 (There are also suffixed versions of the increment and decrement
805 macros, for situations where the full generality of these basic macros
806 can be exchanged for some performance.)
807 However, the way a programmer should think about references is not so
809 much in terms of the bare reference count, but in terms of ownership of
810 references. A reference to an xV can be owned by any of a variety of
811 entities: another xV, the Perl interpreter, an XS data structure, a
812 piece of running code, or a dynamic scope. An xV generally does not
813 know what entities own the references to it; it only knows how many
814 references there are, which is the reference count.
815 To correctly maintain reference counts, it is essential to keep track
817 of what references the XS code is manipulating. The programmer should
818 always know where a reference has come from and who owns it, and be
819 aware of any creation or destruction of references, and any transfers
820 of ownership. Because ownership isn't represented explicitly in the xV
821 data structures, only the reference count need be actually maintained
822 by the code, and that means that this understanding of ownership is not
823 actually evident in the code. For example, transferring ownership of a
824 reference from one owner to another doesn't change the reference count
825 at all, so may be achieved with no actual code. (The transferring code
826 doesn't touch the referenced object, but does need to ensure that the
827 former owner knows that it no longer owns the reference, and that the
828 new owner knows that it now does.)
829 An xV that is visible at the Perl level should not become unreferenced
831 and thus be destroyed. Normally, an object will only become
832 unreferenced when it is no longer visible, often by the same means that
833 makes it invisible. For example, a Perl reference value (RV) owns a
834 reference to its referent, so if the RV is overwritten that reference
835 gets destroyed, and the no-longer-reachable referent may be destroyed
836 as a result.
837 Many functions have some kind of reference manipulation as part of
839 their purpose. Sometimes this is documented in terms of ownership of
840 references, and sometimes it is (less helpfully) documented in terms of
841 changes to reference counts. For example, the newRV_inc() function is
842 documented to create a new RV (with reference count 1) and increment
843 the reference count of the referent that was supplied by the caller.
844 This is best understood as creating a new reference to the referent,
845 which is owned by the created RV, and returning to the caller ownership
846 of the sole reference to the RV. The newRV_noinc() function instead
847 does not increment the reference count of the referent, but the RV
848 nevertheless ends up owning a reference to the referent. It is
849 therefore implied that the caller of "newRV_noinc()" is relinquishing a
850 reference to the referent, making this conceptually a more complicated
851 operation even though it does less to the data structures.
852 For example, imagine you want to return a reference from an XSUB
854 function. Inside the XSUB routine, you create an SV which initially
855 has just a single reference, owned by the XSUB routine. This reference
856 needs to be disposed of before the routine is complete, otherwise it
857 will leak, preventing the SV from ever being destroyed. So to create
858 an RV referencing the SV, it is most convenient to pass the SV to
859 "newRV_noinc()", which consumes that reference. Now the XSUB routine
860 no longer owns a reference to the SV, but does own a reference to the
861 RV, which in turn owns a reference to the SV. The ownership of the
862 reference to the RV is then transferred by the process of returning the
863 RV from the XSUB.
864 There are some convenience functions available that can help with the
866 destruction of xVs. These functions introduce the concept of
867 "mortality". Much documentation speaks of an xV itself being mortal,
868 but this is misleading. It is really a reference to an xV that is
869 mortal, and it is possible for there to be more than one mortal
870 reference to a single xV. For a reference to be mortal means that it
871 is owned by the temps stack, one of perl's many internal stacks, which
872 will destroy that reference "a short time later". Usually the "short
873 time later" is the end of the current Perl statement. However, it gets
874 more complicated around dynamic scopes: there can be multiple sets of
875 mortal references hanging around at the same time, with different death
876 dates. Internally, the actual determinant for when mortal xV
877 references are destroyed depends on two macros, SAVETMPS and FREETMPS.
878 See perlcall and perlxs for more details on these macros.
879 Mortal references are mainly used for xVs that are placed on perl's
881 main stack. The stack is problematic for reference tracking, because
882 it contains a lot of xV references, but doesn't own those references:
883 they are not counted. Currently, there are many bugs resulting from
884 xVs being destroyed while referenced by the stack, because the stack's
885 uncounted references aren't enough to keep the xVs alive. So when
886 putting an (uncounted) reference on the stack, it is vitally important
887 to ensure that there will be a counted reference to the same xV that
888 will last at least as long as the uncounted reference. But it's also
889 important that that counted reference be cleaned up at an appropriate
890 time, and not unduly prolong the xV's life. For there to be a mortal
891 reference is often the best way to satisfy this requirement, especially
892 if the xV was created especially to be put on the stack and would
893 otherwise be unreferenced.
894 To create a mortal reference, use the functions:
896 SV* sv_newmortal()
898 SV* sv_mortalcopy(SV*)
899 SV* sv_2mortal(SV*)
900 "sv_newmortal()" creates an SV (with the undefined value) whose sole
902 reference is mortal. "sv_mortalcopy()" creates an xV whose value is a
903 copy of a supplied xV and whose sole reference is mortal.
904 "sv_2mortal()" mortalises an existing xV reference: it transfers
905 ownership of a reference from the caller to the temps stack. Because
906 "sv_newmortal" gives the new SV no value, it must normally be given one
907 via "sv_setpv", "sv_setiv", etc. :
908 SV *tmp = sv_newmortal();
910 sv_setiv(tmp, an_integer);
911 As that is multiple C statements it is quite common so see this idiom
913 instead:
914 SV *tmp = sv_2mortal(newSViv(an_integer));
916 The mortal routines are not just for SVs; AVs and HVs can be made
918 mortal by passing their address (type-casted to "SV*") to the
919 "sv_2mortal" or "sv_mortalcopy" routines.
920 Stashes and Globs
922 A stash is a hash that contains all variables that are defined within a
923 package. Each key of the stash is a symbol name (shared by all the
924 different types of objects that have the same name), and each value in
925 the hash table is a GV (Glob Value). This GV in turn contains
926 references to the various objects of that name, including (but not
927 limited to) the following:
928 Scalar Value
930 Array Value
931 Hash Value
932 I/O Handle
933 Format
934 Subroutine
935 There is a single stash called "PL_defstash" that holds the items that
937 exist in the "main" package. To get at the items in other packages,
938 append the string "::" to the package name. The items in the "Foo"
939 package are in the stash "Foo::" in PL_defstash. The items in the
940 "Bar::Baz" package are in the stash "Baz::" in "Bar::"'s stash.
941 To get the stash pointer for a particular package, use the function:
943 HV* gv_stashpv(const char* name, I32 flags)
945 HV* gv_stashsv(SV*, I32 flags)
946 The first function takes a literal string, the second uses the string
948 stored in the SV. Remember that a stash is just a hash table, so you
949 get back an "HV*". The "flags" flag will create a new package if it is
950 set to GV_ADD.
951 The name that "gv_stash*v" wants is the name of the package whose
953 symbol table you want. The default package is called "main". If you
954 have multiply nested packages, pass their names to "gv_stash*v",
955 separated by "::" as in the Perl language itself.
956 Alternately, if you have an SV that is a blessed reference, you can
958 find out the stash pointer by using:
959 HV* SvSTASH(SvRV(SV*));
961 then use the following to get the package name itself:
963 char* HvNAME(HV* stash);
965 If you need to bless or re-bless an object you can use the following
967 function:
968 SV* sv_bless(SV*, HV* stash)
970 where the first argument, an "SV*", must be a reference, and the second
972 argument is a stash. The returned "SV*" can now be used in the same
973 way as any other SV.
974 For more information on references and blessings, consult perlref.
976 Double-Typed SVs
978 Scalar variables normally contain only one type of value, an integer,
979 double, pointer, or reference. Perl will automatically convert the
980 actual scalar data from the stored type into the requested type.
981 Some scalar variables contain more than one type of scalar data. For
983 example, the variable $! contains either the numeric value of "errno"
984 or its string equivalent from either "strerror" or "sys_errlist[]".
985 To force multiple data values into an SV, you must do two things: use
987 the "sv_set*v" routines to add the additional scalar type, then set a
988 flag so that Perl will believe it contains more than one type of data.
989 The four macros to set the flags are:
990 SvIOK_on
992 SvNOK_on
993 SvPOK_on
994 SvROK_on
995 The particular macro you must use depends on which "sv_set*v" routine
997 you called first. This is because every "sv_set*v" routine turns on
998 only the bit for the particular type of data being set, and turns off
999 all the rest.
1000 For example, to create a new Perl variable called "dberror" that
1002 contains both the numeric and descriptive string error values, you
1003 could use the following code:
1004 extern int dberror;
1006 extern char *dberror_list;
1007 SV* sv = get_sv("dberror", GV_ADD);
1009 sv_setiv(sv, (IV) dberror);
1010 sv_setpv(sv, dberror_list[dberror]);
1011 SvIOK_on(sv);
1012 If the order of "sv_setiv" and "sv_setpv" had been reversed, then the
1014 macro "SvPOK_on" would need to be called instead of "SvIOK_on".
1015 Read-Only Values
1017 In Perl 5.16 and earlier, copy-on-write (see the next section) shared a
1018 flag bit with read-only scalars. So the only way to test whether
1019 "sv_setsv", etc., will raise a "Modification of a read-only value"
1020 error in those versions is:
1021 SvREADONLY(sv) && !SvIsCOW(sv)
1023 Under Perl 5.18 and later, SvREADONLY only applies to read-only
1025 variables, and, under 5.20, copy-on-write scalars can also be read-
1026 only, so the above check is incorrect. You just want:
1027 SvREADONLY(sv)
1029 If you need to do this check often, define your own macro like this:
1031 #if PERL_VERSION >= 18
1033 # define SvTRULYREADONLY(sv) SvREADONLY(sv)
1034 #else
1035 # define SvTRULYREADONLY(sv) (SvREADONLY(sv) && !SvIsCOW(sv))
1036 #endif
1037 Copy on Write
1039 Perl implements a copy-on-write (COW) mechanism for scalars, in which
1040 string copies are not immediately made when requested, but are deferred
1041 until made necessary by one or the other scalar changing. This is
1042 mostly transparent, but one must take care not to modify string buffers
1043 that are shared by multiple SVs.
1044 You can test whether an SV is using copy-on-write with "SvIsCOW(sv)".
1046 You can force an SV to make its own copy of its string buffer by
1048 calling "sv_force_normal(sv)" or SvPV_force_nolen(sv).
1049 If you want to make the SV drop its string buffer, use
1051 "sv_force_normal_flags(sv, SV_COW_DROP_PV)" or simply "sv_setsv(sv,
1052 NULL)".
1053 All of these functions will croak on read-only scalars (see the
1055 previous section for more on those).
1056 To test that your code is behaving correctly and not modifying COW
1058 buffers, on systems that support mmap(2) (i.e., Unix) you can configure
1059 perl with "-Accflags=-DPERL_DEBUG_READONLY_COW" and it will turn buffer
1060 violations into crashes. You will find it to be marvellously slow, so
1061 you may want to skip perl's own tests.
1062 Magic Variables
1064 [This section still under construction. Ignore everything here. Post
1065 no bills. Everything not permitted is forbidden.]
1066 Any SV may be magical, that is, it has special features that a normal
1068 SV does not have. These features are stored in the SV structure in a
1069 linked list of "struct magic"'s, typedef'ed to "MAGIC".
1070 struct magic {
1072 MAGIC* mg_moremagic;
1073 MGVTBL* mg_virtual;
1074 U16 mg_private;
1075 char mg_type;
1076 U8 mg_flags;
1077 I32 mg_len;
1078 SV* mg_obj;
1079 char* mg_ptr;
1080 };
1081 Note this is current as of patchlevel 0, and could change at any time.
1083 Assigning Magic
1085 Perl adds magic to an SV using the sv_magic function:
1086 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
1088 The "sv" argument is a pointer to the SV that is to acquire a new
1090 magical feature.
1091 If "sv" is not already magical, Perl uses the "SvUPGRADE" macro to
1093 convert "sv" to type "SVt_PVMG". Perl then continues by adding new
1094 magic to the beginning of the linked list of magical features. Any
1095 prior entry of the same type of magic is deleted. Note that this can
1096 be overridden, and multiple instances of the same type of magic can be
1097 associated with an SV.
1098 The "name" and "namlen" arguments are used to associate a string with
1100 the magic, typically the name of a variable. "namlen" is stored in the
1101 "mg_len" field and if "name" is non-null then either a "savepvn" copy
1102 of "name" or "name" itself is stored in the "mg_ptr" field, depending
1103 on whether "namlen" is greater than zero or equal to zero respectively.
1104 As a special case, if "(name && namlen == HEf_SVKEY)" then "name" is
1105 assumed to contain an "SV*" and is stored as-is with its REFCNT
1106 incremented.
1107 The sv_magic function uses "how" to determine which, if any, predefined
1109 "Magic Virtual Table" should be assigned to the "mg_virtual" field.
1110 See the "Magic Virtual Tables" section below. The "how" argument is
1111 also stored in the "mg_type" field. The value of "how" should be
1112 chosen from the set of macros "PERL_MAGIC_foo" found in perl.h. Note
1113 that before these macros were added, Perl internals used to directly
1114 use character literals, so you may occasionally come across old code or
1115 documentation referring to 'U' magic rather than "PERL_MAGIC_uvar" for
1116 example.
1117 The "obj" argument is stored in the "mg_obj" field of the "MAGIC"
1119 structure. If it is not the same as the "sv" argument, the reference
1120 count of the "obj" object is incremented. If it is the same, or if the
1121 "how" argument is "PERL_MAGIC_arylen", "PERL_MAGIC_regdatum",
1122 "PERL_MAGIC_regdata", or if it is a NULL pointer, then "obj" is merely
1123 stored, without the reference count being incremented.
1124 See also "sv_magicext" in perlapi for a more flexible way to add magic
1126 to an SV.
1127 There is also a function to add magic to an "HV":
1129 void hv_magic(HV *hv, GV *gv, int how);
1131 This simply calls "sv_magic" and coerces the "gv" argument into an
1133 "SV".
1134 To remove the magic from an SV, call the function sv_unmagic:
1136 int sv_unmagic(SV *sv, int type);
1138 The "type" argument should be equal to the "how" value when the "SV"
1140 was initially made magical.
1141 However, note that "sv_unmagic" removes all magic of a certain "type"
1143 from the "SV". If you want to remove only certain magic of a "type"
1144 based on the magic virtual table, use "sv_unmagicext" instead:
1145 int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);
1147 Magic Virtual Tables
1149 The "mg_virtual" field in the "MAGIC" structure is a pointer to an
1150 "MGVTBL", which is a structure of function pointers and stands for
1151 "Magic Virtual Table" to handle the various operations that might be
1152 applied to that variable.
1153 The "MGVTBL" has five (or sometimes eight) pointers to the following
1155 routine types:
1156 int (*svt_get) (pTHX_ SV* sv, MAGIC* mg);
1158 int (*svt_set) (pTHX_ SV* sv, MAGIC* mg);
1159 U32 (*svt_len) (pTHX_ SV* sv, MAGIC* mg);
1160 int (*svt_clear)(pTHX_ SV* sv, MAGIC* mg);
1161 int (*svt_free) (pTHX_ SV* sv, MAGIC* mg);
1162 int (*svt_copy) (pTHX_ SV *sv, MAGIC* mg, SV *nsv,
1164 const char *name, I32 namlen);
1165 int (*svt_dup) (pTHX_ MAGIC *mg, CLONE_PARAMS *param);
1166 int (*svt_local)(pTHX_ SV *nsv, MAGIC *mg);
1167 This MGVTBL structure is set at compile-time in perl.h and there are
1169 currently 32 types. These different structures contain pointers to
1170 various routines that perform additional actions depending on which
1171 function is being called.
1172 Function pointer Action taken
1174 ---------------- ------------
1175 svt_get Do something before the value of the SV is
1176 retrieved.
1177 svt_set Do something after the SV is assigned a value.
1178 svt_len Report on the SV's length.
1179 svt_clear Clear something the SV represents.
1180 svt_free Free any extra storage associated with the SV.
1181 svt_copy copy tied variable magic to a tied element
1183 svt_dup duplicate a magic structure during thread cloning
1184 svt_local copy magic to local value during 'local'
1185 For instance, the MGVTBL structure called "vtbl_sv" (which corresponds
1187 to an "mg_type" of "PERL_MAGIC_sv") contains:
1188 { magic_get, magic_set, magic_len, 0, 0 }
1190 Thus, when an SV is determined to be magical and of type
1192 "PERL_MAGIC_sv", if a get operation is being performed, the routine
1193 "magic_get" is called. All the various routines for the various
1194 magical types begin with "magic_". NOTE: the magic routines are not
1195 considered part of the Perl API, and may not be exported by the Perl
1196 library.
1197 The last three slots are a recent addition, and for source code
1199 compatibility they are only checked for if one of the three flags
1200 MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags. This means that
1201 most code can continue declaring a vtable as a 5-element value. These
1202 three are currently used exclusively by the threading code, and are
1203 highly subject to change.
1204 The current kinds of Magic Virtual Tables are:
1206 mg_type
1208 (old-style char and macro) MGVTBL Type of magic
1209 -------------------------- ------ -------------
1210 \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
1211 # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
1212 % PERL_MAGIC_rhash (none) Extra data for restricted
1213 hashes
1214 * PERL_MAGIC_debugvar vtbl_debugvar $DB::single, signal, trace
1215 vars
1216 . PERL_MAGIC_pos vtbl_pos pos() lvalue
1217 : PERL_MAGIC_symtab (none) Extra data for symbol
1218 tables
1219 < PERL_MAGIC_backref vtbl_backref For weak ref data
1220 @ PERL_MAGIC_arylen_p (none) To move arylen out of XPVAV
1221 B PERL_MAGIC_bm vtbl_regexp Boyer-Moore
1222 (fast string search)
1223 c PERL_MAGIC_overload_table vtbl_ovrld Holds overload table
1224 (AMT) on stash
1225 D PERL_MAGIC_regdata vtbl_regdata Regex match position data
1226 (@+ and @- vars)
1227 d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
1228 element
1229 E PERL_MAGIC_env vtbl_env %ENV hash
1230 e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
1231 f PERL_MAGIC_fm vtbl_regexp Formline
1232 ('compiled' format)
1233 g PERL_MAGIC_regex_global vtbl_mglob m//g target
1234 H PERL_MAGIC_hints vtbl_hints %^H hash
1235 h PERL_MAGIC_hintselem vtbl_hintselem %^H hash element
1236 I PERL_MAGIC_isa vtbl_isa @ISA array
1237 i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
1238 k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
1239 L PERL_MAGIC_dbfile (none) Debugger %_<filename
1240 l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename
1241 element
1242 N PERL_MAGIC_shared (none) Shared between threads
1243 n PERL_MAGIC_shared_scalar (none) Shared between threads
1244 o PERL_MAGIC_collxfrm vtbl_collxfrm Locale transformation
1245 P PERL_MAGIC_tied vtbl_pack Tied array or hash
1246 p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
1247 q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
1248 r PERL_MAGIC_qr vtbl_regexp Precompiled qr// regex
1249 S PERL_MAGIC_sig (none) %SIG hash
1250 s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
1251 t PERL_MAGIC_taint vtbl_taint Taintedness
1252 U PERL_MAGIC_uvar vtbl_uvar Available for use by
1253 extensions
1254 u PERL_MAGIC_uvar_elem (none) Reserved for use by
1255 extensions
1256 V PERL_MAGIC_vstring (none) SV was vstring literal
1257 v PERL_MAGIC_vec vtbl_vec vec() lvalue
1258 w PERL_MAGIC_utf8 vtbl_utf8 Cached UTF-8 information
1259 x PERL_MAGIC_substr vtbl_substr substr() lvalue
1260 Y PERL_MAGIC_nonelem vtbl_nonelem Array element that does not
1261 exist
1262 y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
1263 variable / smart parameter
1264 vivification
1265 \ PERL_MAGIC_lvref vtbl_lvref Lvalue reference
1266 constructor
1267 ] PERL_MAGIC_checkcall vtbl_checkcall Inlining/mutation of call
1268 to this CV
1269 ~ PERL_MAGIC_ext (none) Available for use by
1270 extensions
1271 When an uppercase and lowercase letter both exist in the table, then
1273 the uppercase letter is typically used to represent some kind of
1274 composite type (a list or a hash), and the lowercase letter is used to
1275 represent an element of that composite type. Some internals code makes
1276 use of this case relationship. However, 'v' and 'V' (vec and v-string)
1277 are in no way related.
1278 The "PERL_MAGIC_ext" and "PERL_MAGIC_uvar" magic types are defined
1280 specifically for use by extensions and will not be used by perl itself.
1281 Extensions can use "PERL_MAGIC_ext" magic to 'attach' private
1282 information to variables (typically objects). This is especially
1283 useful because there is no way for normal perl code to corrupt this
1284 private information (unlike using extra elements of a hash object).
1285 Similarly, "PERL_MAGIC_uvar" magic can be used much like tie() to call
1287 a C function any time a scalar's value is used or changed. The
1288 "MAGIC"'s "mg_ptr" field points to a "ufuncs" structure:
1289 struct ufuncs {
1291 I32 (*uf_val)(pTHX_ IV, SV*);
1292 I32 (*uf_set)(pTHX_ IV, SV*);
1293 IV uf_index;
1294 };
1295 When the SV is read from or written to, the "uf_val" or "uf_set"
1297 function will be called with "uf_index" as the first arg and a pointer
1298 to the SV as the second. A simple example of how to add
1299 "PERL_MAGIC_uvar" magic is shown below. Note that the ufuncs structure
1300 is copied by sv_magic, so you can safely allocate it on the stack.
1301 void
1303 Umagic(sv)
1304 SV *sv;
1305 PREINIT:
1306 struct ufuncs uf;
1307 CODE:
1308 uf.uf_val = &my_get_fn;
1309 uf.uf_set = &my_set_fn;
1310 uf.uf_index = 0;
1311 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
1312 Attaching "PERL_MAGIC_uvar" to arrays is permissible but has no effect.
1314 For hashes there is a specialized hook that gives control over hash
1316 keys (but not values). This hook calls "PERL_MAGIC_uvar" 'get' magic
1317 if the "set" function in the "ufuncs" structure is NULL. The hook is
1318 activated whenever the hash is accessed with a key specified as an "SV"
1319 through the functions "hv_store_ent", "hv_fetch_ent", "hv_delete_ent",
1320 and "hv_exists_ent". Accessing the key as a string through the
1321 functions without the "..._ent" suffix circumvents the hook. See
1322 "GUTS" in Hash::Util::FieldHash for a detailed description.
1323 Note that because multiple extensions may be using "PERL_MAGIC_ext" or
1325 "PERL_MAGIC_uvar" magic, it is important for extensions to take extra
1326 care to avoid conflict. Typically only using the magic on objects
1327 blessed into the same class as the extension is sufficient. For
1328 "PERL_MAGIC_ext" magic, it is usually a good idea to define an
1329 "MGVTBL", even if all its fields will be 0, so that individual "MAGIC"
1330 pointers can be identified as a particular kind of magic using their
1331 magic virtual table. "mg_findext" provides an easy way to do that:
1332 STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };
1334 MAGIC *mg;
1336 if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
1337 /* this is really ours, not another module's PERL_MAGIC_ext */
1338 my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
1339 ...
1340 }
1341 Also note that the "sv_set*()" and "sv_cat*()" functions described
1343 earlier do not invoke 'set' magic on their targets. This must be done
1344 by the user either by calling the "SvSETMAGIC()" macro after calling
1345 these functions, or by using one of the "sv_set*_mg()" or
1346 "sv_cat*_mg()" functions. Similarly, generic C code must call the
1347 "SvGETMAGIC()" macro to invoke any 'get' magic if they use an SV
1348 obtained from external sources in functions that don't handle magic.
1349 See perlapi for a description of these functions. For example, calls
1350 to the "sv_cat*()" functions typically need to be followed by
1351 "SvSETMAGIC()", but they don't need a prior "SvGETMAGIC()" since their
1352 implementation handles 'get' magic.
1353 Finding Magic
1355 MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that
1356 * type */
1357 This routine returns a pointer to a "MAGIC" structure stored in the SV.
1359 If the SV does not have that magical feature, "NULL" is returned. If
1360 the SV has multiple instances of that magical feature, the first one
1361 will be returned. "mg_findext" can be used to find a "MAGIC" structure
1362 of an SV based on both its magic type and its magic virtual table:
1363 MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);
1365 Also, if the SV passed to "mg_find" or "mg_findext" is not of type
1367 SVt_PVMG, Perl may core dump.
1368 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
1370 This routine checks to see what types of magic "sv" has. If the
1372 mg_type field is an uppercase letter, then the mg_obj is copied to
1373 "nsv", but the mg_type field is changed to be the lowercase letter.
1374 Understanding the Magic of Tied Hashes and Arrays
1376 Tied hashes and arrays are magical beasts of the "PERL_MAGIC_tied"
1377 magic type.
1378 WARNING: As of the 5.004 release, proper usage of the array and hash
1380 access functions requires understanding a few caveats. Some of these
1381 caveats are actually considered bugs in the API, to be fixed in later
1382 releases, and are bracketed with [MAYCHANGE] below. If you find
1383 yourself actually applying such information in this section, be aware
1384 that the behavior may change in the future, umm, without warning.
1385 The perl tie function associates a variable with an object that
1387 implements the various GET, SET, etc methods. To perform the
1388 equivalent of the perl tie function from an XSUB, you must mimic this
1389 behaviour. The code below carries out the necessary steps -- firstly
1390 it creates a new hash, and then creates a second hash which it blesses
1391 into the class which will implement the tie methods. Lastly it ties
1392 the two hashes together, and returns a reference to the new tied hash.
1393 Note that the code below does NOT call the TIEHASH method in the MyTie
1394 class - see "Calling Perl Routines from within C Programs" for details
1395 on how to do this.
1396 SV*
1398 mytie()
1399 PREINIT:
1400 HV *hash;
1401 HV *stash;
1402 SV *tie;
1403 CODE:
1404 hash = newHV();
1405 tie = newRV_noinc((SV*)newHV());
1406 stash = gv_stashpv("MyTie", GV_ADD);
1407 sv_bless(tie, stash);
1408 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1409 RETVAL = newRV_noinc(hash);
1410 OUTPUT:
1411 RETVAL
1412 The "av_store" function, when given a tied array argument, merely
1414 copies the magic of the array onto the value to be "stored", using
1415 "mg_copy". It may also return NULL, indicating that the value did not
1416 actually need to be stored in the array. [MAYCHANGE] After a call to
1417 "av_store" on a tied array, the caller will usually need to call
1418 "mg_set(val)" to actually invoke the perl level "STORE" method on the
1419 TIEARRAY object. If "av_store" did return NULL, a call to
1420 "SvREFCNT_dec(val)" will also be usually necessary to avoid a memory
1421 leak. [/MAYCHANGE]
1422 The previous paragraph is applicable verbatim to tied hash access using
1424 the "hv_store" and "hv_store_ent" functions as well.
1425 "av_fetch" and the corresponding hash functions "hv_fetch" and
1427 "hv_fetch_ent" actually return an undefined mortal value whose magic
1428 has been initialized using "mg_copy". Note the value so returned does
1429 not need to be deallocated, as it is already mortal. [MAYCHANGE] But
1430 you will need to call "mg_get()" on the returned value in order to
1431 actually invoke the perl level "FETCH" method on the underlying TIE
1432 object. Similarly, you may also call "mg_set()" on the return value
1433 after possibly assigning a suitable value to it using "sv_setsv",
1434 which will invoke the "STORE" method on the TIE object. [/MAYCHANGE]
1435 [MAYCHANGE] In other words, the array or hash fetch/store functions
1437 don't really fetch and store actual values in the case of tied arrays
1438 and hashes. They merely call "mg_copy" to attach magic to the values
1439 that were meant to be "stored" or "fetched". Later calls to "mg_get"
1440 and "mg_set" actually do the job of invoking the TIE methods on the
1441 underlying objects. Thus the magic mechanism currently implements a
1442 kind of lazy access to arrays and hashes.
1443 Currently (as of perl version 5.004), use of the hash and array access
1445 functions requires the user to be aware of whether they are operating
1446 on "normal" hashes and arrays, or on their tied variants. The API may
1447 be changed to provide more transparent access to both tied and normal
1448 data types in future versions. [/MAYCHANGE]
1449 You would do well to understand that the TIEARRAY and TIEHASH
1451 interfaces are mere sugar to invoke some perl method calls while using
1452 the uniform hash and array syntax. The use of this sugar imposes some
1453 overhead (typically about two to four extra opcodes per FETCH/STORE
1454 operation, in addition to the creation of all the mortal variables
1455 required to invoke the methods). This overhead will be comparatively
1456 small if the TIE methods are themselves substantial, but if they are
1457 only a few statements long, the overhead will not be insignificant.
1458 Localizing changes
1460 Perl has a very handy construction
1461 {
1463 local $var = 2;
1464 ...
1465 }
1466 This construction is approximately equivalent to
1468 {
1470 my $oldvar = $var;
1471 $var = 2;
1472 ...
1473 $var = $oldvar;
1474 }
1475 The biggest difference is that the first construction would reinstate
1477 the initial value of $var, irrespective of how control exits the block:
1478 "goto", "return", "die"/"eval", etc. It is a little bit more efficient
1479 as well.
1480 There is a way to achieve a similar task from C via Perl API: create a
1482 pseudo-block, and arrange for some changes to be automatically undone
1483 at the end of it, either explicit, or via a non-local exit (via die()).
1484 A block-like construct is created by a pair of "ENTER"/"LEAVE" macros
1485 (see "Returning a Scalar" in perlcall). Such a construct may be
1486 created specially for some important localized task, or an existing one
1487 (like boundaries of enclosing Perl subroutine/block, or an existing
1488 pair for freeing TMPs) may be used. (In the second case the overhead
1489 of additional localization must be almost negligible.) Note that any
1490 XSUB is automatically enclosed in an "ENTER"/"LEAVE" pair.
1491 Inside such a pseudo-block the following service is available:
1493 "SAVEINT(int i)"
1495 "SAVEIV(IV i)"
1496 "SAVEI32(I32 i)"
1497 "SAVELONG(long i)"
1498 These macros arrange things to restore the value of integer
1499 variable "i" at the end of enclosing pseudo-block.
1500 SAVESPTR(s)
1502 SAVEPPTR(p)
1503 These macros arrange things to restore the value of pointers "s"
1504 and "p". "s" must be a pointer of a type which survives conversion
1505 to "SV*" and back, "p" should be able to survive conversion to
1506 "char*" and back.
1507 "SAVEFREESV(SV *sv)"
1509 The refcount of "sv" will be decremented at the end of pseudo-
1510 block. This is similar to "sv_2mortal" in that it is also a
1511 mechanism for doing a delayed "SvREFCNT_dec". However, while
1512 "sv_2mortal" extends the lifetime of "sv" until the beginning of
1513 the next statement, "SAVEFREESV" extends it until the end of the
1514 enclosing scope. These lifetimes can be wildly different.
1515 Also compare "SAVEMORTALIZESV".
1517 "SAVEMORTALIZESV(SV *sv)"
1519 Just like "SAVEFREESV", but mortalizes "sv" at the end of the
1520 current scope instead of decrementing its reference count. This
1521 usually has the effect of keeping "sv" alive until the statement
1522 that called the currently live scope has finished executing.
1523 "SAVEFREEOP(OP *op)"
1525 The "OP *" is op_free()ed at the end of pseudo-block.
1526 SAVEFREEPV(p)
1528 The chunk of memory which is pointed to by "p" is Safefree()ed at
1529 the end of pseudo-block.
1530 "SAVECLEARSV(SV *sv)"
1532 Clears a slot in the current scratchpad which corresponds to "sv"
1533 at the end of pseudo-block.
1534 "SAVEDELETE(HV *hv, char *key, I32 length)"
1536 The key "key" of "hv" is deleted at the end of pseudo-block. The
1537 string pointed to by "key" is Safefree()ed. If one has a key in
1538 short-lived storage, the corresponding string may be reallocated
1539 like this:
1540 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
1542 "SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)"
1544 At the end of pseudo-block the function "f" is called with the only
1545 argument "p".
1546 "SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)"
1548 At the end of pseudo-block the function "f" is called with the
1549 implicit context argument (if any), and "p".
1550 "SAVESTACK_POS()"
1552 The current offset on the Perl internal stack (cf. "SP") is
1553 restored at the end of pseudo-block.
1554 The following API list contains functions, thus one needs to provide
1556 pointers to the modifiable data explicitly (either C pointers, or
1557 Perlish "GV *"s). Where the above macros take "int", a similar
1558 function takes "int *".
1559 "SV* save_scalar(GV *gv)"
1561 Equivalent to Perl code "local $gv".
1562 "AV* save_ary(GV *gv)"
1564 "HV* save_hash(GV *gv)"
1565 Similar to "save_scalar", but localize @gv and %gv.
1566 "void save_item(SV *item)"
1568 Duplicates the current value of "SV", on the exit from the current
1569 "ENTER"/"LEAVE" pseudo-block will restore the value of "SV" using
1570 the stored value. It doesn't handle magic. Use "save_scalar" if
1571 magic is affected.
1572 "void save_list(SV **sarg, I32 maxsarg)"
1574 A variant of "save_item" which takes multiple arguments via an
1575 array "sarg" of "SV*" of length "maxsarg".
1576 "SV* save_svref(SV **sptr)"
1578 Similar to "save_scalar", but will reinstate an "SV *".
1579 "void save_aptr(AV **aptr)"
1581 "void save_hptr(HV **hptr)"
1582 Similar to "save_svref", but localize "AV *" and "HV *".
1583 The "Alias" module implements localization of the basic types within
1585 the caller's scope. People who are interested in how to localize
1586 things in the containing scope should take a look there too.
1587
1589 XSUBs and the Argument Stack
1590 The XSUB mechanism is a simple way for Perl programs to access C
1591 subroutines. An XSUB routine will have a stack that contains the
1592 arguments from the Perl program, and a way to map from the Perl data
1593 structures to a C equivalent.
1594 The stack arguments are accessible through the ST(n) macro, which
1596 returns the "n"'th stack argument. Argument 0 is the first argument
1597 passed in the Perl subroutine call. These arguments are "SV*", and can
1598 be used anywhere an "SV*" is used.
1599 Most of the time, output from the C routine can be handled through use
1601 of the RETVAL and OUTPUT directives. However, there are some cases
1602 where the argument stack is not already long enough to handle all the
1603 return values. An example is the POSIX tzname() call, which takes no
1604 arguments, but returns two, the local time zone's standard and summer
1605 time abbreviations.
1606 To handle this situation, the PPCODE directive is used and the stack is
1608 extended using the macro:
1609 EXTEND(SP, num);
1611 where "SP" is the macro that represents the local copy of the stack
1613 pointer, and "num" is the number of elements the stack should be
1614 extended by.
1615 Now that there is room on the stack, values can be pushed on it using
1617 "PUSHs" macro. The pushed values will often need to be "mortal" (See
1618 "Reference Counts and Mortality"):
1619 PUSHs(sv_2mortal(newSViv(an_integer)))
1621 PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
1622 PUSHs(sv_2mortal(newSVnv(a_double)))
1623 PUSHs(sv_2mortal(newSVpv("Some String",0)))
1624 /* Although the last example is better written as the more
1625 * efficient: */
1626 PUSHs(newSVpvs_flags("Some String", SVs_TEMP))
1627 And now the Perl program calling "tzname", the two values will be
1629 assigned as in:
1630 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1632 An alternate (and possibly simpler) method to pushing values on the
1634 stack is to use the macro:
1635 XPUSHs(SV*)
1637 This macro automatically adjusts the stack for you, if needed. Thus,
1639 you do not need to call "EXTEND" to extend the stack.
1640 Despite their suggestions in earlier versions of this document the
1642 macros "(X)PUSH[iunp]" are not suited to XSUBs which return multiple
1643 results. For that, either stick to the "(X)PUSHs" macros shown above,
1644 or use the new "m(X)PUSH[iunp]" macros instead; see "Putting a C value
1645 on Perl stack".
1646 For more information, consult perlxs and perlxstut.
1648 Autoloading with XSUBs
1650 If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts
1651 the fully-qualified name of the autoloaded subroutine in the $AUTOLOAD
1652 variable of the XSUB's package.
1653 But it also puts the same information in certain fields of the XSUB
1655 itself:
1656 HV *stash = CvSTASH(cv);
1658 const char *subname = SvPVX(cv);
1659 STRLEN name_length = SvCUR(cv); /* in bytes */
1660 U32 is_utf8 = SvUTF8(cv);
1661 "SvPVX(cv)" contains just the sub name itself, not including the
1663 package. For an AUTOLOAD routine in UNIVERSAL or one of its
1664 superclasses, "CvSTASH(cv)" returns NULL during a method call on a
1665 nonexistent package.
1666 Note: Setting $AUTOLOAD stopped working in 5.6.1, which did not support
1668 XS AUTOLOAD subs at all. Perl 5.8.0 introduced the use of fields in
1669 the XSUB itself. Perl 5.16.0 restored the setting of $AUTOLOAD. If
1670 you need to support 5.8-5.14, use the XSUB's fields.
1671 Calling Perl Routines from within C Programs
1673 There are four routines that can be used to call a Perl subroutine from
1674 within a C program. These four are:
1675 I32 call_sv(SV*, I32);
1677 I32 call_pv(const char*, I32);
1678 I32 call_method(const char*, I32);
1679 I32 call_argv(const char*, I32, char**);
1680 The routine most often used is "call_sv". The "SV*" argument contains
1682 either the name of the Perl subroutine to be called, or a reference to
1683 the subroutine. The second argument consists of flags that control the
1684 context in which the subroutine is called, whether or not the
1685 subroutine is being passed arguments, how errors should be trapped, and
1686 how to treat return values.
1687 All four routines return the number of arguments that the subroutine
1689 returned on the Perl stack.
1690 These routines used to be called "perl_call_sv", etc., before Perl
1692 v5.6.0, but those names are now deprecated; macros of the same name are
1693 provided for compatibility.
1694 When using any of these routines (except "call_argv"), the programmer
1696 must manipulate the Perl stack. These include the following macros and
1697 functions:
1698 dSP
1700 SP
1701 PUSHMARK()
1702 PUTBACK
1703 SPAGAIN
1704 ENTER
1705 SAVETMPS
1706 FREETMPS
1707 LEAVE
1708 XPUSH*()
1709 POP*()
1710 For a detailed description of calling conventions from C to Perl,
1712 consult perlcall.
1713 Putting a C value on Perl stack
1715 A lot of opcodes (this is an elementary operation in the internal perl
1716 stack machine) put an SV* on the stack. However, as an optimization
1717 the corresponding SV is (usually) not recreated each time. The opcodes
1718 reuse specially assigned SVs (targets) which are (as a corollary) not
1719 constantly freed/created.
1720 Each of the targets is created only once (but see "Scratchpads and
1722 recursion" below), and when an opcode needs to put an integer, a
1723 double, or a string on stack, it just sets the corresponding parts of
1724 its target and puts the target on stack.
1725 The macro to put this target on stack is "PUSHTARG", and it is directly
1727 used in some opcodes, as well as indirectly in zillions of others,
1728 which use it via "(X)PUSH[iunp]".
1729 Because the target is reused, you must be careful when pushing multiple
1731 values on the stack. The following code will not do what you think:
1732 XPUSHi(10);
1734 XPUSHi(20);
1735 This translates as "set "TARG" to 10, push a pointer to "TARG" onto the
1737 stack; set "TARG" to 20, push a pointer to "TARG" onto the stack". At
1738 the end of the operation, the stack does not contain the values 10 and
1739 20, but actually contains two pointers to "TARG", which we have set to
1740 20.
1741 If you need to push multiple different values then you should either
1743 use the "(X)PUSHs" macros, or else use the new "m(X)PUSH[iunp]" macros,
1744 none of which make use of "TARG". The "(X)PUSHs" macros simply push an
1745 SV* on the stack, which, as noted under "XSUBs and the Argument Stack",
1746 will often need to be "mortal". The new "m(X)PUSH[iunp]" macros make
1747 this a little easier to achieve by creating a new mortal for you (via
1748 "(X)PUSHmortal"), pushing that onto the stack (extending it if
1749 necessary in the case of the "mXPUSH[iunp]" macros), and then setting
1750 its value. Thus, instead of writing this to "fix" the example above:
1751 XPUSHs(sv_2mortal(newSViv(10)))
1753 XPUSHs(sv_2mortal(newSViv(20)))
1754 you can simply write:
1756 mXPUSHi(10)
1758 mXPUSHi(20)
1759 On a related note, if you do use "(X)PUSH[iunp]", then you're going to
1761 need a "dTARG" in your variable declarations so that the "*PUSH*"
1762 macros can make use of the local variable "TARG". See also "dTARGET"
1763 and "dXSTARG".
1764 Scratchpads
1766 The question remains on when the SVs which are targets for opcodes are
1767 created. The answer is that they are created when the current unit--a
1768 subroutine or a file (for opcodes for statements outside of
1769 subroutines)--is compiled. During this time a special anonymous Perl
1770 array is created, which is called a scratchpad for the current unit.
1771 A scratchpad keeps SVs which are lexicals for the current unit and are
1773 targets for opcodes. A previous version of this document stated that
1774 one can deduce that an SV lives on a scratchpad by looking on its
1775 flags: lexicals have "SVs_PADMY" set, and targets have "SVs_PADTMP"
1776 set. But this has never been fully true. "SVs_PADMY" could be set on
1777 a variable that no longer resides in any pad. While targets do have
1778 "SVs_PADTMP" set, it can also be set on variables that have never
1779 resided in a pad, but nonetheless act like targets. As of perl 5.21.5,
1780 the "SVs_PADMY" flag is no longer used and is defined as 0.
1781 "SvPADMY()" now returns true for anything without "SVs_PADTMP".
1782 The correspondence between OPs and targets is not 1-to-1. Different
1784 OPs in the compile tree of the unit can use the same target, if this
1785 would not conflict with the expected life of the temporary.
1786 Scratchpads and recursion
1788 In fact it is not 100% true that a compiled unit contains a pointer to
1789 the scratchpad AV. In fact it contains a pointer to an AV of
1790 (initially) one element, and this element is the scratchpad AV. Why do
1791 we need an extra level of indirection?
1792 The answer is recursion, and maybe threads. Both these can create
1794 several execution pointers going into the same subroutine. For the
1795 subroutine-child not write over the temporaries for the subroutine-
1796 parent (lifespan of which covers the call to the child), the parent and
1797 the child should have different scratchpads. (And the lexicals should
1798 be separate anyway!)
1799 So each subroutine is born with an array of scratchpads (of length 1).
1801 On each entry to the subroutine it is checked that the current depth of
1802 the recursion is not more than the length of this array, and if it is,
1803 new scratchpad is created and pushed into the array.
1804 The targets on this scratchpad are "undef"s, but they are already
1806 marked with correct flags.
1807
1809 Allocation
1810 All memory meant to be used with the Perl API functions should be
1811 manipulated using the macros described in this section. The macros
1812 provide the necessary transparency between differences in the actual
1813 malloc implementation that is used within perl.
1814 It is suggested that you enable the version of malloc that is
1816 distributed with Perl. It keeps pools of various sizes of unallocated
1817 memory in order to satisfy allocation requests more quickly. However,
1818 on some platforms, it may cause spurious malloc or free errors.
1819 The following three macros are used to initially allocate memory :
1821 Newx(pointer, number, type);
1823 Newxc(pointer, number, type, cast);
1824 Newxz(pointer, number, type);
1825 The first argument "pointer" should be the name of a variable that will
1827 point to the newly allocated memory.
1828 The second and third arguments "number" and "type" specify how many of
1830 the specified type of data structure should be allocated. The argument
1831 "type" is passed to "sizeof". The final argument to "Newxc", "cast",
1832 should be used if the "pointer" argument is different from the "type"
1833 argument.
1834 Unlike the "Newx" and "Newxc" macros, the "Newxz" macro calls "memzero"
1836 to zero out all the newly allocated memory.
1837 Reallocation
1839 Renew(pointer, number, type);
1840 Renewc(pointer, number, type, cast);
1841 Safefree(pointer)
1842 These three macros are used to change a memory buffer size or to free a
1844 piece of memory no longer needed. The arguments to "Renew" and
1845 "Renewc" match those of "New" and "Newc" with the exception of not
1846 needing the "magic cookie" argument.
1847 Moving
1849 Move(source, dest, number, type);
1850 Copy(source, dest, number, type);
1851 Zero(dest, number, type);
1852 These three macros are used to move, copy, or zero out previously
1854 allocated memory. The "source" and "dest" arguments point to the
1855 source and destination starting points. Perl will move, copy, or zero
1856 out "number" instances of the size of the "type" data structure (using
1857 the "sizeof" function).
1858
1860 The most recent development releases of Perl have been experimenting
1861 with removing Perl's dependency on the "normal" standard I/O suite and
1862 allowing other stdio implementations to be used. This involves
1863 creating a new abstraction layer that then calls whichever
1864 implementation of stdio Perl was compiled with. All XSUBs should now
1865 use the functions in the PerlIO abstraction layer and not make any
1866 assumptions about what kind of stdio is being used.
1867 For a complete description of the PerlIO abstraction, consult perlapio.
1869
1871 Code tree
1872 Here we describe the internal form your code is converted to by Perl.
1873 Start with a simple example:
1874 $a = $b + $c;
1876 This is converted to a tree similar to this one:
1878 assign-to
1880 / \
1881 + $a
1882 / \
1883 $b $c
1884 (but slightly more complicated). This tree reflects the way Perl
1886 parsed your code, but has nothing to do with the execution order.
1887 There is an additional "thread" going through the nodes of the tree
1888 which shows the order of execution of the nodes. In our simplified
1889 example above it looks like:
1890 $b ---> $c ---> + ---> $a ---> assign-to
1892 But with the actual compile tree for "$a = $b + $c" it is different:
1894 some nodes optimized away. As a corollary, though the actual tree
1895 contains more nodes than our simplified example, the execution order is
1896 the same as in our example.
1897 Examining the tree
1899 If you have your perl compiled for debugging (usually done with
1900 "-DDEBUGGING" on the "Configure" command line), you may examine the
1901 compiled tree by specifying "-Dx" on the Perl command line. The output
1902 takes several lines per node, and for "$b+$c" it looks like this:
1903 5 TYPE = add ===> 6
1905 TARG = 1
1906 FLAGS = (SCALAR,KIDS)
1907 {
1908 TYPE = null ===> (4)
1909 (was rv2sv)
1910 FLAGS = (SCALAR,KIDS)
1911 {
1912 3 TYPE = gvsv ===> 4
1913 FLAGS = (SCALAR)
1914 GV = main::b
1915 }
1916 }
1917 {
1918 TYPE = null ===> (5)
1919 (was rv2sv)
1920 FLAGS = (SCALAR,KIDS)
1921 {
1922 4 TYPE = gvsv ===> 5
1923 FLAGS = (SCALAR)
1924 GV = main::c
1925 }
1926 }
1927 This tree has 5 nodes (one per "TYPE" specifier), only 3 of them are
1929 not optimized away (one per number in the left column). The immediate
1930 children of the given node correspond to "{}" pairs on the same level
1931 of indentation, thus this listing corresponds to the tree:
1932 add
1934 / \
1935 null null
1936 | |
1937 gvsv gvsv
1938 The execution order is indicated by "===>" marks, thus it is "3 4 5 6"
1940 (node 6 is not included into above listing), i.e., "gvsv gvsv add
1941 whatever".
1942 Each of these nodes represents an op, a fundamental operation inside
1944 the Perl core. The code which implements each operation can be found
1945 in the pp*.c files; the function which implements the op with type
1946 "gvsv" is "pp_gvsv", and so on. As the tree above shows, different ops
1947 have different numbers of children: "add" is a binary operator, as one
1948 would expect, and so has two children. To accommodate the various
1949 different numbers of children, there are various types of op data
1950 structure, and they link together in different ways.
1951 The simplest type of op structure is "OP": this has no children. Unary
1953 operators, "UNOP"s, have one child, and this is pointed to by the
1954 "op_first" field. Binary operators ("BINOP"s) have not only an
1955 "op_first" field but also an "op_last" field. The most complex type of
1956 op is a "LISTOP", which has any number of children. In this case, the
1957 first child is pointed to by "op_first" and the last child by
1958 "op_last". The children in between can be found by iteratively
1959 following the "OpSIBLING" pointer from the first child to the last (but
1960 see below).
1961 There are also some other op types: a "PMOP" holds a regular
1963 expression, and has no children, and a "LOOP" may or may not have
1964 children. If the "op_children" field is non-zero, it behaves like a
1965 "LISTOP". To complicate matters, if a "UNOP" is actually a "null" op
1966 after optimization (see "Compile pass 2: context propagation") it will
1967 still have children in accordance with its former type.
1968 Finally, there is a "LOGOP", or logic op. Like a "LISTOP", this has one
1970 or more children, but it doesn't have an "op_last" field: so you have
1971 to follow "op_first" and then the "OpSIBLING" chain itself to find the
1972 last child. Instead it has an "op_other" field, which is comparable to
1973 the "op_next" field described below, and represents an alternate
1974 execution path. Operators like "and", "or" and "?" are "LOGOP"s. Note
1975 that in general, "op_other" may not point to any of the direct children
1976 of the "LOGOP".
1977 Starting in version 5.21.2, perls built with the experimental define
1979 "-DPERL_OP_PARENT" add an extra boolean flag for each op, "op_moresib".
1980 When not set, this indicates that this is the last op in an "OpSIBLING"
1981 chain. This frees up the "op_sibling" field on the last sibling to
1982 point back to the parent op. Under this build, that field is also
1983 renamed "op_sibparent" to reflect its joint role. The macro
1984 OpSIBLING(o) wraps this special behaviour, and always returns NULL on
1985 the last sibling. With this build the op_parent(o) function can be
1986 used to find the parent of any op. Thus for forward compatibility, you
1987 should always use the OpSIBLING(o) macro rather than accessing
1988 "op_sibling" directly.
1989 Another way to examine the tree is to use a compiler back-end module,
1991 such as B::Concise.
1992 Compile pass 1: check routines
1994 The tree is created by the compiler while yacc code feeds it the
1995 constructions it recognizes. Since yacc works bottom-up, so does the
1996 first pass of perl compilation.
1997 What makes this pass interesting for perl developers is that some
1999 optimization may be performed on this pass. This is optimization by
2000 so-called "check routines". The correspondence between node names and
2001 corresponding check routines is described in opcode.pl (do not forget
2002 to run "make regen_headers" if you modify this file).
2003 A check routine is called when the node is fully constructed except for
2005 the execution-order thread. Since at this time there are no back-links
2006 to the currently constructed node, one can do most any operation to the
2007 top-level node, including freeing it and/or creating new nodes
2008 above/below it.
2009 The check routine returns the node which should be inserted into the
2011 tree (if the top-level node was not modified, check routine returns its
2012 argument).
2013 By convention, check routines have names "ck_*". They are usually
2015 called from "new*OP" subroutines (or "convert") (which in turn are
2016 called from perly.y).
2017 Compile pass 1a: constant folding
2019 Immediately after the check routine is called the returned node is
2020 checked for being compile-time executable. If it is (the value is
2021 judged to be constant) it is immediately executed, and a constant node
2022 with the "return value" of the corresponding subtree is substituted
2023 instead. The subtree is deleted.
2024 If constant folding was not performed, the execution-order thread is
2026 created.
2027 Compile pass 2: context propagation
2029 When a context for a part of compile tree is known, it is propagated
2030 down through the tree. At this time the context can have 5 values
2031 (instead of 2 for runtime context): void, boolean, scalar, list, and
2032 lvalue. In contrast with the pass 1 this pass is processed from top to
2033 bottom: a node's context determines the context for its children.
2034 Additional context-dependent optimizations are performed at this time.
2036 Since at this moment the compile tree contains back-references (via
2037 "thread" pointers), nodes cannot be free()d now. To allow optimized-
2038 away nodes at this stage, such nodes are null()ified instead of
2039 free()ing (i.e. their type is changed to OP_NULL).
2040 Compile pass 3: peephole optimization
2042 After the compile tree for a subroutine (or for an "eval" or a file) is
2043 created, an additional pass over the code is performed. This pass is
2044 neither top-down or bottom-up, but in the execution order (with
2045 additional complications for conditionals). Optimizations performed at
2046 this stage are subject to the same restrictions as in the pass 2.
2047 Peephole optimizations are done by calling the function pointed to by
2049 the global variable "PL_peepp". By default, "PL_peepp" just calls the
2050 function pointed to by the global variable "PL_rpeepp". By default,
2051 that performs some basic op fixups and optimisations along the
2052 execution-order op chain, and recursively calls "PL_rpeepp" for each
2053 side chain of ops (resulting from conditionals). Extensions may
2054 provide additional optimisations or fixups, hooking into either the
2055 per-subroutine or recursive stage, like this:
2056 static peep_t prev_peepp;
2058 static void my_peep(pTHX_ OP *o)
2059 {
2060 /* custom per-subroutine optimisation goes here */
2061 prev_peepp(aTHX_ o);
2062 /* custom per-subroutine optimisation may also go here */
2063 }
2064 BOOT:
2065 prev_peepp = PL_peepp;
2066 PL_peepp = my_peep;
2067 static peep_t prev_rpeepp;
2069 static void my_rpeep(pTHX_ OP *o)
2070 {
2071 OP *orig_o = o;
2072 for(; o; o = o->op_next) {
2073 /* custom per-op optimisation goes here */
2074 }
2075 prev_rpeepp(aTHX_ orig_o);
2076 }
2077 BOOT:
2078 prev_rpeepp = PL_rpeepp;
2079 PL_rpeepp = my_rpeep;
2080 Pluggable runops
2082 The compile tree is executed in a runops function. There are two
2083 runops functions, in run.c and in dump.c. "Perl_runops_debug" is used
2084 with DEBUGGING and "Perl_runops_standard" is used otherwise. For fine
2085 control over the execution of the compile tree it is possible to
2086 provide your own runops function.
2087 It's probably best to copy one of the existing runops functions and
2089 change it to suit your needs. Then, in the BOOT section of your XS
2090 file, add the line:
2091 PL_runops = my_runops;
2093 This function should be as efficient as possible to keep your programs
2095 running as fast as possible.
2096 Compile-time scope hooks
2098 As of perl 5.14 it is possible to hook into the compile-time lexical
2099 scope mechanism using "Perl_blockhook_register". This is used like
2100 this:
2101 STATIC void my_start_hook(pTHX_ int full);
2103 STATIC BHK my_hooks;
2104 BOOT:
2106 BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
2107 Perl_blockhook_register(aTHX_ &my_hooks);
2108 This will arrange to have "my_start_hook" called at the start of
2110 compiling every lexical scope. The available hooks are:
2111 "void bhk_start(pTHX_ int full)"
2113 This is called just after starting a new lexical scope. Note that
2114 Perl code like
2115 if ($x) { ... }
2117 creates two scopes: the first starts at the "(" and has "full ==
2119 1", the second starts at the "{" and has "full == 0". Both end at
2120 the "}", so calls to "start" and "pre"/"post_end" will match.
2121 Anything pushed onto the save stack by this hook will be popped
2122 just before the scope ends (between the "pre_" and "post_end"
2123 hooks, in fact).
2124 "void bhk_pre_end(pTHX_ OP **o)"
2126 This is called at the end of a lexical scope, just before unwinding
2127 the stack. o is the root of the optree representing the scope; it
2128 is a double pointer so you can replace the OP if you need to.
2129 "void bhk_post_end(pTHX_ OP **o)"
2131 This is called at the end of a lexical scope, just after unwinding
2132 the stack. o is as above. Note that it is possible for calls to
2133 "pre_" and "post_end" to nest, if there is something on the save
2134 stack that calls string eval.
2135 "void bhk_eval(pTHX_ OP *const o)"
2137 This is called just before starting to compile an "eval STRING",
2138 "do FILE", "require" or "use", after the eval has been set up. o
2139 is the OP that requested the eval, and will normally be an
2140 "OP_ENTEREVAL", "OP_DOFILE" or "OP_REQUIRE".
2141 Once you have your hook functions, you need a "BHK" structure to put
2143 them in. It's best to allocate it statically, since there is no way to
2144 free it once it's registered. The function pointers should be inserted
2145 into this structure using the "BhkENTRY_set" macro, which will also set
2146 flags indicating which entries are valid. If you do need to allocate
2147 your "BHK" dynamically for some reason, be sure to zero it before you
2148 start.
2149 Once registered, there is no mechanism to switch these hooks off, so if
2151 that is necessary you will need to do this yourself. An entry in "%^H"
2152 is probably the best way, so the effect is lexically scoped; however it
2153 is also possible to use the "BhkDISABLE" and "BhkENABLE" macros to
2154 temporarily switch entries on and off. You should also be aware that
2155 generally speaking at least one scope will have opened before your
2156 extension is loaded, so you will see some "pre"/"post_end" pairs that
2157 didn't have a matching "start".
2158
2160 To aid debugging, the source file dump.c contains a number of functions
2161 which produce formatted output of internal data structures.
2162 The most commonly used of these functions is "Perl_sv_dump"; it's used
2164 for dumping SVs, AVs, HVs, and CVs. The "Devel::Peek" module calls
2165 "sv_dump" to produce debugging output from Perl-space, so users of that
2166 module should already be familiar with its format.
2167 "Perl_op_dump" can be used to dump an "OP" structure or any of its
2169 derivatives, and produces output similar to "perl -Dx"; in fact,
2170 "Perl_dump_eval" will dump the main root of the code being evaluated,
2171 exactly like "-Dx".
2172 Other useful functions are "Perl_dump_sub", which turns a "GV" into an
2174 op tree, "Perl_dump_packsubs" which calls "Perl_dump_sub" on all the
2175 subroutines in a package like so: (Thankfully, these are all xsubs, so
2176 there is no op tree)
2177 (gdb) print Perl_dump_packsubs(PL_defstash)
2179 SUB attributes::bootstrap = (xsub 0x811fedc 0)
2181 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
2183 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
2185 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
2187 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
2189 and "Perl_dump_all", which dumps all the subroutines in the stash and
2191 the op tree of the main root.
2192
2194 Background and PERL_IMPLICIT_CONTEXT
2195 The Perl interpreter can be regarded as a closed box: it has an API for
2196 feeding it code or otherwise making it do things, but it also has
2197 functions for its own use. This smells a lot like an object, and there
2198 are ways for you to build Perl so that you can have multiple
2199 interpreters, with one interpreter represented either as a C structure,
2200 or inside a thread-specific structure. These structures contain all
2201 the context, the state of that interpreter.
2202 One macro controls the major Perl build flavor: MULTIPLICITY. The
2204 MULTIPLICITY build has a C structure that packages all the interpreter
2205 state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
2206 normally defined, and enables the support for passing in a "hidden"
2207 first argument that represents all three data structures. MULTIPLICITY
2208 makes multi-threaded perls possible (with the ithreads threading model,
2209 related to the macro USE_ITHREADS.)
2210 Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
2212 PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
2213 former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the
2214 internal variables of Perl to be wrapped inside a single global struct,
2215 struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or the
2216 function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes one step
2217 further, there is still a single struct (allocated in main() either
2218 from heap or from stack) but there are no global data symbols pointing
2219 to it. In either case the global struct should be initialized as the
2220 very first thing in main() using Perl_init_global_struct() and
2221 correspondingly tear it down after perl_free() using
2222 Perl_free_global_struct(), please see miniperlmain.c for usage details.
2223 You may also need to use "dVAR" in your coding to "declare the global
2224 variables" when you are using them. dTHX does this for you
2225 automatically.
2226 To see whether you have non-const data you can use a BSD (or GNU)
2228 compatible "nm":
2229 nm libperl.a | grep -v ' [TURtr] '
2231 If this displays any "D" or "d" symbols (or possibly "C" or "c"), you
2233 have non-const data. The symbols the "grep" removed are as follows:
2234 "Tt" are text, or code, the "Rr" are read-only (const) data, and the
2235 "U" is <undefined>, external symbols referred to.
2236 The test t/porting/libperl.t does this kind of symbol sanity checking
2238 on "libperl.a".
2239 For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
2241 doesn't actually hide all symbols inside a big global struct: some
2242 PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE
2243 then hides everything (see how the PERLIO_FUNCS_DECL is used).
2244 All this obviously requires a way for the Perl internal functions to be
2246 either subroutines taking some kind of structure as the first argument,
2247 or subroutines taking nothing as the first argument. To enable these
2248 two very different ways of building the interpreter, the Perl source
2249 (as it does in so many other situations) makes heavy use of macros and
2250 subroutine naming conventions.
2251 First problem: deciding which functions will be public API functions
2253 and which will be private. All functions whose names begin "S_" are
2254 private (think "S" for "secret" or "static"). All other functions
2255 begin with "Perl_", but just because a function begins with "Perl_"
2256 does not mean it is part of the API. (See "Internal Functions".) The
2257 easiest way to be sure a function is part of the API is to find its
2258 entry in perlapi. If it exists in perlapi, it's part of the API. If
2259 it doesn't, and you think it should be (i.e., you need it for your
2260 extension), send mail via perlbug explaining why you think it should
2261 be.
2262 Second problem: there must be a syntax so that the same subroutine
2264 declarations and calls can pass a structure as their first argument, or
2265 pass nothing. To solve this, the subroutines are named and declared in
2266 a particular way. Here's a typical start of a static function used
2267 within the Perl guts:
2268 STATIC void
2270 S_incline(pTHX_ char *s)
2271 STATIC becomes "static" in C, and may be #define'd to nothing in some
2273 configurations in the future.
2274 A public function (i.e. part of the internal API, but not necessarily
2276 sanctioned for use in extensions) begins like this:
2277 void
2279 Perl_sv_setiv(pTHX_ SV* dsv, IV num)
2280 "pTHX_" is one of a number of macros (in perl.h) that hide the details
2282 of the interpreter's context. THX stands for "thread", "this", or
2283 "thingy", as the case may be. (And no, George Lucas is not involved.
2284 :-) The first character could be 'p' for a prototype, 'a' for argument,
2285 or 'd' for declaration, so we have "pTHX", "aTHX" and "dTHX", and their
2286 variants.
2287 When Perl is built without options that set PERL_IMPLICIT_CONTEXT,
2289 there is no first argument containing the interpreter's context. The
2290 trailing underscore in the pTHX_ macro indicates that the macro
2291 expansion needs a comma after the context argument because other
2292 arguments follow it. If PERL_IMPLICIT_CONTEXT is not defined, pTHX_
2293 will be ignored, and the subroutine is not prototyped to take the extra
2294 argument. The form of the macro without the trailing underscore is
2295 used when there are no additional explicit arguments.
2296 When a core function calls another, it must pass the context. This is
2298 normally hidden via macros. Consider "sv_setiv". It expands into
2299 something like this:
2300 #ifdef PERL_IMPLICIT_CONTEXT
2302 #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
2303 /* can't do this for vararg functions, see below */
2304 #else
2305 #define sv_setiv Perl_sv_setiv
2306 #endif
2307 This works well, and means that XS authors can gleefully write:
2309 sv_setiv(foo, bar);
2311 and still have it work under all the modes Perl could have been
2313 compiled with.
2314 This doesn't work so cleanly for varargs functions, though, as macros
2316 imply that the number of arguments is known in advance. Instead we
2317 either need to spell them out fully, passing "aTHX_" as the first
2318 argument (the Perl core tends to do this with functions like
2319 Perl_warner), or use a context-free version.
2320 The context-free version of Perl_warner is called
2322 Perl_warner_nocontext, and does not take the extra argument. Instead
2323 it does dTHX; to get the context from thread-local storage. We
2324 "#define warner Perl_warner_nocontext" so that extensions get source
2325 compatibility at the expense of performance. (Passing an arg is
2326 cheaper than grabbing it from thread-local storage.)
2327 You can ignore [pad]THXx when browsing the Perl headers/sources. Those
2329 are strictly for use within the core. Extensions and embedders need
2330 only be aware of [pad]THX.
2331 So what happened to dTHR?
2333 "dTHR" was introduced in perl 5.005 to support the older thread model.
2334 The older thread model now uses the "THX" mechanism to pass context
2335 pointers around, so "dTHR" is not useful any more. Perl 5.6.0 and
2336 later still have it for backward source compatibility, but it is
2337 defined to be a no-op.
2338 How do I use all this in extensions?
2340 When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call any
2341 functions in the Perl API will need to pass the initial context
2342 argument somehow. The kicker is that you will need to write it in such
2343 a way that the extension still compiles when Perl hasn't been built
2344 with PERL_IMPLICIT_CONTEXT enabled.
2345 There are three ways to do this. First, the easy but inefficient way,
2347 which is also the default, in order to maintain source compatibility
2348 with extensions: whenever XSUB.h is #included, it redefines the aTHX
2349 and aTHX_ macros to call a function that will return the context.
2350 Thus, something like:
2351 sv_setiv(sv, num);
2353 in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
2355 in effect:
2356 Perl_sv_setiv(Perl_get_context(), sv, num);
2358 or to this otherwise:
2360 Perl_sv_setiv(sv, num);
2362 You don't have to do anything new in your extension to get this; since
2364 the Perl library provides Perl_get_context(), it will all just work.
2365 The second, more efficient way is to use the following template for
2367 your Foo.xs:
2368 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2370 #include "EXTERN.h"
2371 #include "perl.h"
2372 #include "XSUB.h"
2373 STATIC void my_private_function(int arg1, int arg2);
2375 STATIC void
2377 my_private_function(int arg1, int arg2)
2378 {
2379 dTHX; /* fetch context */
2380 ... call many Perl API functions ...
2381 }
2382 [... etc ...]
2384 MODULE = Foo PACKAGE = Foo
2386 /* typical XSUB */
2388 void
2390 my_xsub(arg)
2391 int arg
2392 CODE:
2393 my_private_function(arg, 10);
2394 Note that the only two changes from the normal way of writing an
2396 extension is the addition of a "#define PERL_NO_GET_CONTEXT" before
2397 including the Perl headers, followed by a "dTHX;" declaration at the
2398 start of every function that will call the Perl API. (You'll know
2399 which functions need this, because the C compiler will complain that
2400 there's an undeclared identifier in those functions.) No changes are
2401 needed for the XSUBs themselves, because the XS() macro is correctly
2402 defined to pass in the implicit context if needed.
2403 The third, even more efficient way is to ape how it is done within the
2405 Perl guts:
2406 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2408 #include "EXTERN.h"
2409 #include "perl.h"
2410 #include "XSUB.h"
2411 /* pTHX_ only needed for functions that call Perl API */
2413 STATIC void my_private_function(pTHX_ int arg1, int arg2);
2414 STATIC void
2416 my_private_function(pTHX_ int arg1, int arg2)
2417 {
2418 /* dTHX; not needed here, because THX is an argument */
2419 ... call Perl API functions ...
2420 }
2421 [... etc ...]
2423 MODULE = Foo PACKAGE = Foo
2425 /* typical XSUB */
2427 void
2429 my_xsub(arg)
2430 int arg
2431 CODE:
2432 my_private_function(aTHX_ arg, 10);
2433 This implementation never has to fetch the context using a function
2435 call, since it is always passed as an extra argument. Depending on
2436 your needs for simplicity or efficiency, you may mix the previous two
2437 approaches freely.
2438 Never add a comma after "pTHX" yourself--always use the form of the
2440 macro with the underscore for functions that take explicit arguments,
2441 or the form without the argument for functions with no explicit
2442 arguments.
2443 If one is compiling Perl with the "-DPERL_GLOBAL_STRUCT" the "dVAR"
2445 definition is needed if the Perl global variables (see perlvars.h or
2446 globvar.sym) are accessed in the function and "dTHX" is not used (the
2447 "dTHX" includes the "dVAR" if necessary). One notices the need for
2448 "dVAR" only with the said compile-time define, because otherwise the
2449 Perl global variables are visible as-is.
2450 Should I do anything special if I call perl from multiple threads?
2452 If you create interpreters in one thread and then proceed to call them
2453 in another, you need to make sure perl's own Thread Local Storage (TLS)
2454 slot is initialized correctly in each of those threads.
2455 The "perl_alloc" and "perl_clone" API functions will automatically set
2457 the TLS slot to the interpreter they created, so that there is no need
2458 to do anything special if the interpreter is always accessed in the
2459 same thread that created it, and that thread did not create or call any
2460 other interpreters afterwards. If that is not the case, you have to
2461 set the TLS slot of the thread before calling any functions in the Perl
2462 API on that particular interpreter. This is done by calling the
2463 "PERL_SET_CONTEXT" macro in that thread as the first thing you do:
2464 /* do this before doing anything else with some_perl */
2466 PERL_SET_CONTEXT(some_perl);
2467 ... other Perl API calls on some_perl go here ...
2469 Future Plans and PERL_IMPLICIT_SYS
2471 Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2472 that the interpreter knows about itself and pass it around, so too are
2473 there plans to allow the interpreter to bundle up everything it knows
2474 about the environment it's running on. This is enabled with the
2475 PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS on
2476 Windows.
2477 This allows the ability to provide an extra pointer (called the "host"
2479 environment) for all the system calls. This makes it possible for all
2480 the system stuff to maintain their own state, broken down into seven C
2481 structures. These are thin wrappers around the usual system calls (see
2482 win32/perllib.c) for the default perl executable, but for a more
2483 ambitious host (like the one that would do fork() emulation) all the
2484 extra work needed to pretend that different interpreters are actually
2485 different "processes", would be done here.
2486 The Perl engine/interpreter and the host are orthogonal entities.
2488 There could be one or more interpreters in a process, and one or more
2489 "hosts", with free association between them.
2490
2492 All of Perl's internal functions which will be exposed to the outside
2493 world are prefixed by "Perl_" so that they will not conflict with XS
2494 functions or functions used in a program in which Perl is embedded.
2495 Similarly, all global variables begin with "PL_". (By convention,
2496 static functions start with "S_".)
2497 Inside the Perl core ("PERL_CORE" defined), you can get at the
2499 functions either with or without the "Perl_" prefix, thanks to a bunch
2500 of defines that live in embed.h. Note that extension code should not
2501 set "PERL_CORE"; this exposes the full perl internals, and is likely to
2502 cause breakage of the XS in each new perl release.
2503 The file embed.h is generated automatically from embed.pl and
2505 embed.fnc. embed.pl also creates the prototyping header files for the
2506 internal functions, generates the documentation and a lot of other bits
2507 and pieces. It's important that when you add a new function to the
2508 core or change an existing one, you change the data in the table in
2509 embed.fnc as well. Here's a sample entry from that table:
2510 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
2512 The second column is the return type, the third column the name.
2514 Columns after that are the arguments. The first column is a set of
2515 flags:
2516 A This function is a part of the public API. All such functions
2518 should also have 'd', very few do not.
2519 p This function has a "Perl_" prefix; i.e. it is defined as
2521 "Perl_av_fetch".
2522 d This function has documentation using the "apidoc" feature which
2524 we'll look at in a second. Some functions have 'd' but not 'A';
2525 docs are good.
2526 Other available flags are:
2528 s This is a static function and is defined as "STATIC S_whatever", and
2530 usually called within the sources as "whatever(...)".
2531 n This does not need an interpreter context, so the definition has no
2533 "pTHX", and it follows that callers don't use "aTHX". (See
2534 "Background and PERL_IMPLICIT_CONTEXT".)
2535 r This function never returns; "croak", "exit" and friends.
2537 f This function takes a variable number of arguments, "printf" style.
2539 The argument list should end with "...", like this:
2540 Afprd |void |croak |const char* pat|...
2542 M This function is part of the experimental development API, and may
2544 change or disappear without notice.
2545 o This function should not have a compatibility macro to define, say,
2547 "Perl_parse" to "parse". It must be called as "Perl_parse".
2548 x This function isn't exported out of the Perl core.
2550 m This is implemented as a macro.
2552 X This function is explicitly exported.
2554 E This function is visible to extensions included in the Perl core.
2556 b Binary backward compatibility; this function is a macro but also has
2558 a "Perl_" implementation (which is exported).
2559 others
2561 See the comments at the top of "embed.fnc" for others.
2562 If you edit embed.pl or embed.fnc, you will need to run "make
2564 regen_headers" to force a rebuild of embed.h and other auto-generated
2565 files.
2566 Formatted Printing of IVs, UVs, and NVs
2568 If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2569 formatting codes like %d, %ld, %f, you should use the following macros
2570 for portability
2571 IVdf IV in decimal
2573 UVuf UV in decimal
2574 UVof UV in octal
2575 UVxf UV in hexadecimal
2576 NVef NV %e-like
2577 NVff NV %f-like
2578 NVgf NV %g-like
2579 These will take care of 64-bit integers and long doubles. For example:
2581 printf("IV is %"IVdf"\n", iv);
2583 The IVdf will expand to whatever is the correct format for the IVs.
2585 Note that there are different "long doubles": Perl will use whatever
2587 the compiler has.
2588 If you are printing addresses of pointers, use UVxf combined with
2590 PTR2UV(), do not use %lx or %p.
2591 Formatted Printing of "Size_t" and "SSize_t"
2593 The most general way to do this is to cast them to a UV or IV, and
2594 print as in the previous section.
2595 But if you're using "PerlIO_printf()", it's less typing and visual
2597 clutter to use the "%z" length modifier (for siZe):
2598 PerlIO_printf("STRLEN is %zu\n", len);
2600 This modifier is not portable, so its use should be restricted to
2602 "PerlIO_printf()".
2603 Pointer-To-Integer and Integer-To-Pointer
2605 Because pointer size does not necessarily equal integer size, use the
2606 follow macros to do it right.
2607 PTR2UV(pointer)
2609 PTR2IV(pointer)
2610 PTR2NV(pointer)
2611 INT2PTR(pointertotype, integer)
2612 For example:
2614 IV iv = ...;
2616 SV *sv = INT2PTR(SV*, iv);
2617 and
2619 AV *av = ...;
2621 UV uv = PTR2UV(av);
2622 Exception Handling
2624 There are a couple of macros to do very basic exception handling in XS
2625 modules. You have to define "NO_XSLOCKS" before including XSUB.h to be
2626 able to use these macros:
2627 #define NO_XSLOCKS
2629 #include "XSUB.h"
2630 You can use these macros if you call code that may croak, but you need
2632 to do some cleanup before giving control back to Perl. For example:
2633 dXCPT; /* set up necessary variables */
2635 XCPT_TRY_START {
2637 code_that_may_croak();
2638 } XCPT_TRY_END
2639 XCPT_CATCH
2641 {
2642 /* do cleanup here */
2643 XCPT_RETHROW;
2644 }
2645 Note that you always have to rethrow an exception that has been caught.
2647 Using these macros, it is not possible to just catch the exception and
2648 ignore it. If you have to ignore the exception, you have to use the
2649 "call_*" function.
2650 The advantage of using the above macros is that you don't have to setup
2652 an extra function for "call_*", and that using these macros is faster
2653 than using "call_*".
2654 Source Documentation
2656 There's an effort going on to document the internal functions and
2657 automatically produce reference manuals from them -- perlapi is one
2658 such manual which details all the functions which are available to XS
2659 writers. perlintern is the autogenerated manual for the functions
2660 which are not part of the API and are supposedly for internal use only.
2661 Source documentation is created by putting POD comments into the C
2663 source, like this:
2664 /*
2666 =for apidoc sv_setiv
2667 Copies an integer into the given SV. Does not handle 'set' magic. See
2669 L<perlapi/sv_setiv_mg>.
2670 =cut
2672 */
2673 Please try and supply some documentation if you add functions to the
2675 Perl core.
2676 Backwards compatibility
2678 The Perl API changes over time. New functions are added or the
2679 interfaces of existing functions are changed. The "Devel::PPPort"
2680 module tries to provide compatibility code for some of these changes,
2681 so XS writers don't have to code it themselves when supporting multiple
2682 versions of Perl.
2683 "Devel::PPPort" generates a C header file ppport.h that can also be run
2685 as a Perl script. To generate ppport.h, run:
2686 perl -MDevel::PPPort -eDevel::PPPort::WriteFile
2688 Besides checking existing XS code, the script can also be used to
2690 retrieve compatibility information for various API calls using the
2691 "--api-info" command line switch. For example:
2692 % perl ppport.h --api-info=sv_magicext
2694 For details, see "perldoc ppport.h".
2696
2698 Perl 5.6.0 introduced Unicode support. It's important for porters and
2699 XS writers to understand this support and make sure that the code they
2700 write does not corrupt Unicode data.
2701 What is Unicode, anyway?
2703 In the olden, less enlightened times, we all used to use ASCII. Most
2704 of us did, anyway. The big problem with ASCII is that it's American.
2705 Well, no, that's not actually the problem; the problem is that it's not
2706 particularly useful for people who don't use the Roman alphabet. What
2707 used to happen was that particular languages would stick their own
2708 alphabet in the upper range of the sequence, between 128 and 255. Of
2709 course, we then ended up with plenty of variants that weren't quite
2710 ASCII, and the whole point of it being a standard was lost.
2711 Worse still, if you've got a language like Chinese or Japanese that has
2713 hundreds or thousands of characters, then you really can't fit them
2714 into a mere 256, so they had to forget about ASCII altogether, and
2715 build their own systems using pairs of numbers to refer to one
2716 character.
2717 To fix this, some people formed Unicode, Inc. and produced a new
2719 character set containing all the characters you can possibly think of
2720 and more. There are several ways of representing these characters, and
2721 the one Perl uses is called UTF-8. UTF-8 uses a variable number of
2722 bytes to represent a character. You can learn more about Unicode and
2723 Perl's Unicode model in perlunicode.
2724 (On EBCDIC platforms, Perl uses instead UTF-EBCDIC, which is a form of
2726 UTF-8 adapted for EBCDIC platforms. Below, we just talk about UTF-8.
2727 UTF-EBCDIC is like UTF-8, but the details are different. The macros
2728 hide the differences from you, just remember that the particular
2729 numbers and bit patterns presented below will differ in UTF-EBCDIC.)
2730 How can I recognise a UTF-8 string?
2732 You can't. This is because UTF-8 data is stored in bytes just like
2733 non-UTF-8 data. The Unicode character 200, (0xC8 for you hex types)
2734 capital E with a grave accent, is represented by the two bytes
2735 "v196.172". Unfortunately, the non-Unicode string "chr(196).chr(172)"
2736 has that byte sequence as well. So you can't tell just by looking --
2737 this is what makes Unicode input an interesting problem.
2738 In general, you either have to know what you're dealing with, or you
2740 have to guess. The API function "is_utf8_string" can help; it'll tell
2741 you if a string contains only valid UTF-8 characters, and the chances
2742 of a non-UTF-8 string looking like valid UTF-8 become very small very
2743 quickly with increasing string length. On a character-by-character
2744 basis, "isUTF8_CHAR" will tell you whether the current character in a
2745 string is valid UTF-8.
2746 How does UTF-8 represent Unicode characters?
2748 As mentioned above, UTF-8 uses a variable number of bytes to store a
2749 character. Characters with values 0...127 are stored in one byte, just
2750 like good ol' ASCII. Character 128 is stored as "v194.128"; this
2751 continues up to character 191, which is "v194.191". Now we've run out
2752 of bits (191 is binary 10111111) so we move on; character 192 is
2753 "v195.128". And so it goes on, moving to three bytes at character
2754 2048. "Unicode Encodings" in perlunicode has pictures of how this
2755 works.
2756 Assuming you know you're dealing with a UTF-8 string, you can find out
2758 how long the first character in it is with the "UTF8SKIP" macro:
2759 char *utf = "\305\233\340\240\201";
2761 I32 len;
2762 len = UTF8SKIP(utf); /* len is 2 here */
2764 utf += len;
2765 len = UTF8SKIP(utf); /* len is 3 here */
2766 Another way to skip over characters in a UTF-8 string is to use
2768 "utf8_hop", which takes a string and a number of characters to skip
2769 over. You're on your own about bounds checking, though, so don't use
2770 it lightly.
2771 All bytes in a multi-byte UTF-8 character will have the high bit set,
2773 so you can test if you need to do something special with this character
2774 like this (the "UTF8_IS_INVARIANT()" is a macro that tests whether the
2775 byte is encoded as a single byte even in UTF-8):
2776 U8 *utf; /* Initialize this to point to the beginning of the
2778 sequence to convert */
2779 U8 *utf_end; /* Initialize this to 1 beyond the end of the sequence
2780 pointed to by 'utf' */
2781 UV uv; /* Returned code point; note: a UV, not a U8, not a
2782 char */
2783 STRLEN len; /* Returned length of character in bytes */
2784 if (!UTF8_IS_INVARIANT(*utf))
2786 /* Must treat this as UTF-8 */
2787 uv = utf8_to_uvchr_buf(utf, utf_end, &len);
2788 else
2789 /* OK to treat this character as a byte */
2790 uv = *utf;
2791 You can also see in that example that we use "utf8_to_uvchr_buf" to get
2793 the value of the character; the inverse function "uvchr_to_utf8" is
2794 available for putting a UV into UTF-8:
2795 if (!UVCHR_IS_INVARIANT(uv))
2797 /* Must treat this as UTF8 */
2798 utf8 = uvchr_to_utf8(utf8, uv);
2799 else
2800 /* OK to treat this character as a byte */
2801 *utf8++ = uv;
2802 You must convert characters to UVs using the above functions if you're
2804 ever in a situation where you have to match UTF-8 and non-UTF-8
2805 characters. You may not skip over UTF-8 characters in this case. If
2806 you do this, you'll lose the ability to match hi-bit non-UTF-8
2807 characters; for instance, if your UTF-8 string contains "v196.172", and
2808 you skip that character, you can never match a "chr(200)" in a
2809 non-UTF-8 string. So don't do that!
2810 (Note that we don't have to test for invariant characters in the
2812 examples above. The functions work on any well-formed UTF-8 input.
2813 It's just that its faster to avoid the function overhead when it's not
2814 needed.)
2815 How does Perl store UTF-8 strings?
2817 Currently, Perl deals with UTF-8 strings and non-UTF-8 strings slightly
2818 differently. A flag in the SV, "SVf_UTF8", indicates that the string
2819 is internally encoded as UTF-8. Without it, the byte value is the
2820 codepoint number and vice versa. This flag is only meaningful if the
2821 SV is "SvPOK" or immediately after stringification via "SvPV" or a
2822 similar macro. You can check and manipulate this flag with the
2823 following macros:
2824 SvUTF8(sv)
2826 SvUTF8_on(sv)
2827 SvUTF8_off(sv)
2828 This flag has an important effect on Perl's treatment of the string: if
2830 UTF-8 data is not properly distinguished, regular expressions,
2831 "length", "substr" and other string handling operations will have
2832 undesirable (wrong) results.
2833 The problem comes when you have, for instance, a string that isn't
2835 flagged as UTF-8, and contains a byte sequence that could be UTF-8 --
2836 especially when combining non-UTF-8 and UTF-8 strings.
2837 Never forget that the "SVf_UTF8" flag is separate from the PV value;
2839 you need to be sure you don't accidentally knock it off while you're
2840 manipulating SVs. More specifically, you cannot expect to do this:
2841 SV *sv;
2843 SV *nsv;
2844 STRLEN len;
2845 char *p;
2846 p = SvPV(sv, len);
2848 frobnicate(p);
2849 nsv = newSVpvn(p, len);
2850 The "char*" string does not tell you the whole story, and you can't
2852 copy or reconstruct an SV just by copying the string value. Check if
2853 the old SV has the UTF8 flag set (after the "SvPV" call), and act
2854 accordingly:
2855 p = SvPV(sv, len);
2857 is_utf8 = SvUTF8(sv);
2858 frobnicate(p, is_utf8);
2859 nsv = newSVpvn(p, len);
2860 if (is_utf8)
2861 SvUTF8_on(nsv);
2862 In the above, your "frobnicate" function has been changed to be made
2864 aware of whether or not it's dealing with UTF-8 data, so that it can
2865 handle the string appropriately.
2866 Since just passing an SV to an XS function and copying the data of the
2868 SV is not enough to copy the UTF8 flags, even less right is just
2869 passing a "char *" to an XS function.
2870 For full generality, use the "DO_UTF8" macro to see if the string in an
2872 SV is to be treated as UTF-8. This takes into account if the call to
2873 the XS function is being made from within the scope of "use bytes". If
2874 so, the underlying bytes that comprise the UTF-8 string are to be
2875 exposed, rather than the character they represent. But this pragma
2876 should only really be used for debugging and perhaps low-level testing
2877 at the byte level. Hence most XS code need not concern itself with
2878 this, but various areas of the perl core do need to support it.
2879 And this isn't the whole story. Starting in Perl v5.12, strings that
2881 aren't encoded in UTF-8 may also be treated as Unicode under various
2882 conditions (see "ASCII Rules versus Unicode Rules" in perlunicode).
2883 This is only really a problem for characters whose ordinals are between
2884 128 and 255, and their behavior varies under ASCII versus Unicode rules
2885 in ways that your code cares about (see "The "Unicode Bug"" in
2886 perlunicode). There is no published API for dealing with this, as it
2887 is subject to change, but you can look at the code for "pp_lc" in pp.c
2888 for an example as to how it's currently done.
2889 How do I convert a string to UTF-8?
2891 If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to
2892 upgrade the non-UTF-8 strings to UTF-8. If you've got an SV, the
2893 easiest way to do this is:
2894 sv_utf8_upgrade(sv);
2896 However, you must not do this, for example:
2898 if (!SvUTF8(left))
2900 sv_utf8_upgrade(left);
2901 If you do this in a binary operator, you will actually change one of
2903 the strings that came into the operator, and, while it shouldn't be
2904 noticeable by the end user, it can cause problems in deficient code.
2905 Instead, "bytes_to_utf8" will give you a UTF-8-encoded copy of its
2907 string argument. This is useful for having the data available for
2908 comparisons and so on, without harming the original SV. There's also
2909 "utf8_to_bytes" to go the other way, but naturally, this will fail if
2910 the string contains any characters above 255 that can't be represented
2911 in a single byte.
2912 How do I compare strings?
2914 "sv_cmp" in perlapi and "sv_cmp_flags" in perlapi do a lexigraphic
2915 comparison of two SV's, and handle UTF-8ness properly. Note, however,
2916 that Unicode specifies a much fancier mechanism for collation,
2917 available via the Unicode::Collate module.
2918 To just compare two strings for equality/non-equality, you can just use
2920 "memEQ()" and "memNE()" as usual, except the strings must be both UTF-8
2921 or not UTF-8 encoded.
2922 To compare two strings case-insensitively, use "foldEQ_utf8()" (the
2924 strings don't have to have the same UTF-8ness).
2925 Is there anything else I need to know?
2927 Not really. Just remember these things:
2928 · There's no way to tell if a "char *" or "U8 *" string is UTF-8 or
2930 not. But you can tell if an SV is to be treated as UTF-8 by calling
2931 "DO_UTF8" on it, after stringifying it with "SvPV" or a similar
2932 macro. And, you can tell if SV is actually UTF-8 (even if it is not
2933 to be treated as such) by looking at its "SvUTF8" flag (again after
2934 stringifying it). Don't forget to set the flag if something should
2935 be UTF-8. Treat the flag as part of the PV, even though it's not --
2936 if you pass on the PV to somewhere, pass on the flag too.
2937 · If a string is UTF-8, always use "utf8_to_uvchr_buf" to get at the
2939 value, unless "UTF8_IS_INVARIANT(*s)" in which case you can use *s.
2940 · When writing a character UV to a UTF-8 string, always use
2942 "uvchr_to_utf8", unless "UVCHR_IS_INVARIANT(uv))" in which case you
2943 can use "*s = uv".
2944 · Mixing UTF-8 and non-UTF-8 strings is tricky. Use "bytes_to_utf8"
2946 to get a new string which is UTF-8 encoded, and then combine them.
2947
2949 Custom operator support is an experimental feature that allows you to
2950 define your own ops. This is primarily to allow the building of
2951 interpreters for other languages in the Perl core, but it also allows
2952 optimizations through the creation of "macro-ops" (ops which perform
2953 the functions of multiple ops which are usually executed together, such
2954 as "gvsv, gvsv, add".)
2955 This feature is implemented as a new op type, "OP_CUSTOM". The Perl
2957 core does not "know" anything special about this op type, and so it
2958 will not be involved in any optimizations. This also means that you
2959 can define your custom ops to be any op structure -- unary, binary,
2960 list and so on -- you like.
2961 It's important to know what custom operators won't do for you. They
2963 won't let you add new syntax to Perl, directly. They won't even let
2964 you add new keywords, directly. In fact, they won't change the way
2965 Perl compiles a program at all. You have to do those changes yourself,
2966 after Perl has compiled the program. You do this either by
2967 manipulating the op tree using a "CHECK" block and the "B::Generate"
2968 module, or by adding a custom peephole optimizer with the "optimize"
2969 module.
2970 When you do this, you replace ordinary Perl ops with custom ops by
2972 creating ops with the type "OP_CUSTOM" and the "op_ppaddr" of your own
2973 PP function. This should be defined in XS code, and should look like
2974 the PP ops in "pp_*.c". You are responsible for ensuring that your op
2975 takes the appropriate number of values from the stack, and you are
2976 responsible for adding stack marks if necessary.
2977 You should also "register" your op with the Perl interpreter so that it
2979 can produce sensible error and warning messages. Since it is possible
2980 to have multiple custom ops within the one "logical" op type
2981 "OP_CUSTOM", Perl uses the value of "o->op_ppaddr" to determine which
2982 custom op it is dealing with. You should create an "XOP" structure for
2983 each ppaddr you use, set the properties of the custom op with
2984 "XopENTRY_set", and register the structure against the ppaddr using
2985 "Perl_custom_op_register". A trivial example might look like:
2986 static XOP my_xop;
2988 static OP *my_pp(pTHX);
2989 BOOT:
2991 XopENTRY_set(&my_xop, xop_name, "myxop");
2992 XopENTRY_set(&my_xop, xop_desc, "Useless custom op");
2993 Perl_custom_op_register(aTHX_ my_pp, &my_xop);
2994 The available fields in the structure are:
2996 xop_name
2998 A short name for your op. This will be included in some error
2999 messages, and will also be returned as "$op->name" by the B module,
3000 so it will appear in the output of module like B::Concise.
3001 xop_desc
3003 A short description of the function of the op.
3004 xop_class
3006 Which of the various *OP structures this op uses. This should be
3007 one of the "OA_*" constants from op.h, namely
3008 OA_BASEOP
3010 OA_UNOP
3011 OA_BINOP
3012 OA_LOGOP
3013 OA_LISTOP
3014 OA_PMOP
3015 OA_SVOP
3016 OA_PADOP
3017 OA_PVOP_OR_SVOP
3018 This should be interpreted as '"PVOP"' only. The "_OR_SVOP" is
3019 because the only core "PVOP", "OP_TRANS", can sometimes be a
3020 "SVOP" instead.
3021 OA_LOOP
3023 OA_COP
3024 The other "OA_*" constants should not be used.
3026 xop_peep
3028 This member is of type "Perl_cpeep_t", which expands to "void
3029 (*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop)". If it is set, this
3030 function will be called from "Perl_rpeep" when ops of this type are
3031 encountered by the peephole optimizer. o is the OP that needs
3032 optimizing; oldop is the previous OP optimized, whose "op_next"
3033 points to o.
3034 "B::Generate" directly supports the creation of custom ops by name.
3036
3038 Note: this section describes a non-public internal API that is subject
3039 to change without notice.
3040 Introduction to the context stack
3042 In Perl, dynamic scoping refers to the runtime nesting of things like
3043 subroutine calls, evals etc, as well as the entering and exiting of
3044 block scopes. For example, the restoring of a "local"ised variable is
3045 determined by the dynamic scope.
3046 Perl tracks the dynamic scope by a data structure called the context
3048 stack, which is an array of "PERL_CONTEXT" structures, and which is
3049 itself a big union for all the types of context. Whenever a new scope
3050 is entered (such as a block, a "for" loop, or a subroutine call), a new
3051 context entry is pushed onto the stack. Similarly when leaving a block
3052 or returning from a subroutine call etc. a context is popped. Since the
3053 context stack represents the current dynamic scope, it can be searched.
3054 For example, "next LABEL" searches back through the stack looking for a
3055 loop context that matches the label; "return" pops contexts until it
3056 finds a sub or eval context or similar; "caller" examines sub contexts
3057 on the stack.
3058 Each context entry is labelled with a context type, "cx_type". Typical
3060 context types are "CXt_SUB", "CXt_EVAL" etc., as well as "CXt_BLOCK"
3061 and "CXt_NULL" which represent a basic scope (as pushed by "pp_enter")
3062 and a sort block. The type determines which part of the context union
3063 are valid.
3064 The main division in the context struct is between a substitution scope
3066 ("CXt_SUBST") and block scopes, which are everything else. The former
3067 is just used while executing "s///e", and won't be discussed further
3068 here.
3069 All the block scope types share a common base, which corresponds to
3071 "CXt_BLOCK". This stores the old values of various scope-related
3072 variables like "PL_curpm", as well as information about the current
3073 scope, such as "gimme". On scope exit, the old variables are restored.
3074 Particular block scope types store extra per-type information. For
3076 example, "CXt_SUB" stores the currently executing CV, while the various
3077 for loop types might hold the original loop variable SV. On scope exit,
3078 the per-type data is processed; for example the CV has its reference
3079 count decremented, and the original loop variable is restored.
3080 The macro "cxstack" returns the base of the current context stack,
3082 while "cxstack_ix" is the index of the current frame within that stack.
3083 In fact, the context stack is actually part of a stack-of-stacks
3085 system; whenever something unusual is done such as calling a "DESTROY"
3086 or tie handler, a new stack is pushed, then popped at the end.
3087 Note that the API described here changed considerably in perl 5.24;
3089 prior to that, big macros like "PUSHBLOCK" and "POPSUB" were used; in
3090 5.24 they were replaced by the inline static functions described below.
3091 In addition, the ordering and detail of how these macros/function work
3092 changed in many ways, often subtly. In particular they didn't handle
3093 saving the savestack and temps stack positions, and required additional
3094 "ENTER", "SAVETMPS" and "LEAVE" compared to the new functions. The old-
3095 style macros will not be described further.
3096 Pushing contexts
3098 For pushing a new context, the two basic functions are "cx =
3099 cx_pushblock()", which pushes a new basic context block and returns its
3100 address, and a family of similar functions with names like
3101 "cx_pushsub(cx)" which populate the additional type-dependent fields in
3102 the "cx" struct. Note that "CXt_NULL" and "CXt_BLOCK" don't have their
3103 own push functions, as they don't store any data beyond that pushed by
3104 "cx_pushblock".
3105 The fields of the context struct and the arguments to the "cx_*"
3107 functions are subject to change between perl releases, representing
3108 whatever is convenient or efficient for that release.
3109 A typical context stack pushing can be found in "pp_entersub"; the
3111 following shows a simplified and stripped-down example of a non-XS
3112 call, along with comments showing roughly what each function does.
3113 dMARK;
3115 U8 gimme = GIMME_V;
3116 bool hasargs = cBOOL(PL_op->op_flags & OPf_STACKED);
3117 OP *retop = PL_op->op_next;
3118 I32 old_ss_ix = PL_savestack_ix;
3119 CV *cv = ....;
3120 /* ... make mortal copies of stack args which are PADTMPs here ... */
3122 /* ... do any additional savestack pushes here ... */
3124 /* Now push a new context entry of type 'CXt_SUB'; initially just
3126 * doing the actions common to all block types: */
3127 cx = cx_pushblock(CXt_SUB, gimme, MARK, old_ss_ix);
3129 /* this does (approximately):
3131 CXINC; /* cxstack_ix++ (grow if necessary) */
3132 cx = CX_CUR(); /* and get the address of new frame */
3133 cx->cx_type = CXt_SUB;
3134 cx->blk_gimme = gimme;
3135 cx->blk_oldsp = MARK - PL_stack_base;
3136 cx->blk_oldsaveix = old_ss_ix;
3137 cx->blk_oldcop = PL_curcop;
3138 cx->blk_oldmarksp = PL_markstack_ptr - PL_markstack;
3139 cx->blk_oldscopesp = PL_scopestack_ix;
3140 cx->blk_oldpm = PL_curpm;
3141 cx->blk_old_tmpsfloor = PL_tmps_floor;
3142 PL_tmps_floor = PL_tmps_ix;
3144 */
3145 /* then update the new context frame with subroutine-specific info,
3148 * such as the CV about to be executed: */
3149 cx_pushsub(cx, cv, retop, hasargs);
3151 /* this does (approximately):
3153 cx->blk_sub.cv = cv;
3154 cx->blk_sub.olddepth = CvDEPTH(cv);
3155 cx->blk_sub.prevcomppad = PL_comppad;
3156 cx->cx_type |= (hasargs) ? CXp_HASARGS : 0;
3157 cx->blk_sub.retop = retop;
3158 SvREFCNT_inc_simple_void_NN(cv);
3159 */
3160 Note that "cx_pushblock()" sets two new floors: for the args stack (to
3162 "MARK") and the temps stack (to "PL_tmps_ix"). While executing at this
3163 scope level, every "nextstate" (amongst others) will reset the args and
3164 tmps stack levels to these floors. Note that since "cx_pushblock" uses
3165 the current value of "PL_tmps_ix" rather than it being passed as an
3166 arg, this dictates at what point "cx_pushblock" should be called. In
3167 particular, any new mortals which should be freed only on scope exit
3168 (rather than at the next "nextstate") should be created first.
3169 Most callers of "cx_pushblock" simply set the new args stack floor to
3171 the top of the previous stack frame, but for "CXt_LOOP_LIST" it stores
3172 the items being iterated over on the stack, and so sets "blk_oldsp" to
3173 the top of these items instead. Note that, contrary to its name,
3174 "blk_oldsp" doesn't always represent the value to restore "PL_stack_sp"
3175 to on scope exit.
3176 Note the early capture of "PL_savestack_ix" to "old_ss_ix", which is
3178 later passed as an arg to "cx_pushblock". In the case of "pp_entersub",
3179 this is because, although most values needing saving are stored in
3180 fields of the context struct, an extra value needs saving only when the
3181 debugger is running, and it doesn't make sense to bloat the struct for
3182 this rare case. So instead it is saved on the savestack. Since this
3183 value gets calculated and saved before the context is pushed, it is
3184 necessary to pass the old value of "PL_savestack_ix" to "cx_pushblock",
3185 to ensure that the saved value gets freed during scope exit. For most
3186 users of "cx_pushblock", where nothing needs pushing on the save stack,
3187 "PL_savestack_ix" is just passed directly as an arg to "cx_pushblock".
3188 Note that where possible, values should be saved in the context struct
3190 rather than on the save stack; it's much faster that way.
3191 Normally "cx_pushblock" should be immediately followed by the
3193 appropriate "cx_pushfoo", with nothing between them; this is because if
3194 code in-between could die (e.g. a warning upgraded to fatal), then the
3195 context stack unwinding code in "dounwind" would see (in the example
3196 above) a "CXt_SUB" context frame, but without all the subroutine-
3197 specific fields set, and crashes would soon ensue.
3198 Where the two must be separate, initially set the type to "CXt_NULL" or
3200 "CXt_BLOCK", and later change it to "CXt_foo" when doing the
3201 "cx_pushfoo". This is exactly what "pp_enteriter" does, once it's
3202 determined which type of loop it's pushing.
3203 Popping contexts
3205 Contexts are popped using "cx_popsub()" etc. and "cx_popblock()". Note
3206 however, that unlike "cx_pushblock", neither of these functions
3207 actually decrement the current context stack index; this is done
3208 separately using "CX_POP()".
3209 There are two main ways that contexts are popped. During normal
3211 execution as scopes are exited, functions like "pp_leave",
3212 "pp_leaveloop" and "pp_leavesub" process and pop just one context using
3213 "cx_popfoo" and "cx_popblock". On the other hand, things like
3214 "pp_return" and "next" may have to pop back several scopes until a sub
3215 or loop context is found, and exceptions (such as "die") need to pop
3216 back contexts until an eval context is found. Both of these are
3217 accomplished by "dounwind()", which is capable of processing and
3218 popping all contexts above the target one.
3219 Here is a typical example of context popping, as found in "pp_leavesub"
3221 (simplified slightly):
3222 U8 gimme;
3224 PERL_CONTEXT *cx;
3225 SV **oldsp;
3226 OP *retop;
3227 cx = CX_CUR();
3229 gimme = cx->blk_gimme;
3231 oldsp = PL_stack_base + cx->blk_oldsp; /* last arg of previous frame */
3232 if (gimme == G_VOID)
3234 PL_stack_sp = oldsp;
3235 else
3236 leave_adjust_stacks(oldsp, oldsp, gimme, 0);
3237 CX_LEAVE_SCOPE(cx);
3239 cx_popsub(cx);
3240 cx_popblock(cx);
3241 retop = cx->blk_sub.retop;
3242 CX_POP(cx);
3243 return retop;
3245 The steps above are in a very specific order, designed to be the
3247 reverse order of when the context was pushed. The first thing to do is
3248 to copy and/or protect any any return arguments and free any temps in
3249 the current scope. Scope exits like an rvalue sub normally return a
3250 mortal copy of their return args (as opposed to lvalue subs). It is
3251 important to make this copy before the save stack is popped or
3252 variables are restored, or bad things like the following can happen:
3253 sub f { my $x =...; $x } # $x freed before we get to copy it
3255 sub f { /(...)/; $1 } # PL_curpm restored before $1 copied
3256 Although we wish to free any temps at the same time, we have to be
3258 careful not to free any temps which are keeping return args alive; nor
3259 to free the temps we have just created while mortal copying return
3260 args. Fortunately, "leave_adjust_stacks()" is capable of making mortal
3261 copies of return args, shifting args down the stack, and only
3262 processing those entries on the temps stack that are safe to do so.
3263 In void context no args are returned, so it's more efficient to skip
3265 calling "leave_adjust_stacks()". Also in void context, a "nextstate" op
3266 is likely to be imminently called which will do a "FREETMPS", so
3267 there's no need to do that either.
3268 The next step is to pop savestack entries: "CX_LEAVE_SCOPE(cx)" is just
3270 defined as "<LEAVE_SCOPE(cx-"blk_oldsaveix)>>. Note that during the
3271 popping, it's possible for perl to call destructors, call "STORE" to
3272 undo localisations of tied vars, and so on. Any of these can die or
3273 call "exit()". In this case, "dounwind()" will be called, and the
3274 current context stack frame will be re-processed. Thus it is vital that
3275 all steps in popping a context are done in such a way to support
3276 reentrancy. The other alternative, of decrementing "cxstack_ix" before
3277 processing the frame, would lead to leaks and the like if something
3278 died halfway through, or overwriting of the current frame.
3279 "CX_LEAVE_SCOPE" itself is safely re-entrant: if only half the
3281 savestack items have been popped before dying and getting trapped by
3282 eval, then the "CX_LEAVE_SCOPE"s in "dounwind" or "pp_leaveeval" will
3283 continue where the first one left off.
3284 The next step is the type-specific context processing; in this case
3286 "cx_popsub". In part, this looks like:
3287 cv = cx->blk_sub.cv;
3289 CvDEPTH(cv) = cx->blk_sub.olddepth;
3290 cx->blk_sub.cv = NULL;
3291 SvREFCNT_dec(cv);
3292 where its processing the just-executed CV. Note that before it
3294 decrements the CV's reference count, it nulls the "blk_sub.cv". This
3295 means that if it re-enters, the CV won't be freed twice. It also means
3296 that you can't rely on such type-specific fields having useful values
3297 after the return from "cx_popfoo".
3298 Next, "cx_popblock" restores all the various interpreter vars to their
3300 previous values or previous high water marks; it expands to:
3301 PL_markstack_ptr = PL_markstack + cx->blk_oldmarksp;
3303 PL_scopestack_ix = cx->blk_oldscopesp;
3304 PL_curpm = cx->blk_oldpm;
3305 PL_curcop = cx->blk_oldcop;
3306 PL_tmps_floor = cx->blk_old_tmpsfloor;
3307 Note that it doesn't restore "PL_stack_sp"; as mentioned earlier, which
3309 value to restore it to depends on the context type (specifically "for
3310 (list) {}"), and what args (if any) it returns; and that will already
3311 have been sorted out earlier by "leave_adjust_stacks()".
3312 Finally, the context stack pointer is actually decremented by
3314 "CX_POP(cx)". After this point, it's possible that that the current
3315 context frame could be overwritten by other contexts being pushed.
3316 Although things like ties and "DESTROY" are supposed to work within a
3317 new context stack, it's best not to assume this. Indeed on debugging
3318 builds, "CX_POP(cx)" deliberately sets "cx" to null to detect code that
3319 is still relying on the field values in that context frame. Note in the
3320 "pp_leavesub()" example above, we grab "blk_sub.retop" before calling
3321 "CX_POP".
3322 Redoing contexts
3324 Finally, there is "cx_topblock(cx)", which acts like a
3325 super-"nextstate" as regards to resetting various vars to their base
3326 values. It is used in places like "pp_next", "pp_redo" and "pp_goto"
3327 where rather than exiting a scope, we want to re-initialise the scope.
3328 As well as resetting "PL_stack_sp" like "nextstate", it also resets
3329 "PL_markstack_ptr", "PL_scopestack_ix" and "PL_curpm". Note that it
3330 doesn't do a "FREETMPS".
3331
3333 Until May 1997, this document was maintained by Jeff Okamoto
3334 <okamoto@corp.hp.com>. It is now maintained as part of Perl itself by
3335 the Perl 5 Porters <perl5-porters@perl.org>.
3336 With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
3338 Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
3339 Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
3340 Stephen McCamant, and Gurusamy Sarathy.
3341
3343 perlapi, perlintern, perlxs, perlembed
3344perl v5.28.2 2018-11-01 PERLGUTS(1)